NMNLC And NM CLFX LP New Mountain Net Lease Corporation And NM CLFX LP [Member] New Mountain Net Lease Corporation And NM CLFX LP Foreign securities Foreign Currency Transactions and Translations Policy [Policy Text Block] Net increase in net assets resulting from operations Net increase in net assets resulting from operations Net increase (decrease) in net assets resulting from operations Net Income (Loss), Including Portion Attributable to Noncontrolling Interest Related Party Transactions [Abstract] PhyNet Dermatology LLC PhyNet Dermatology LLC [Member] PhyNet Dermatology LLC Outstanding balance Long-Term Debt, Gross Permian Holdco 3, Inc. / Permian Trust Permian Holdco 3, Inc. And Permian Trust [Member] Permian Holdco 3, Inc. And Permian Trust Commitment fee percentage Guaranteed Debt, Commitment Fee Guaranteed Debt, Commitment Fee Schedule of Net Assets Investment Company, Changes in Net Assets [Table Text Block] Entity Address, Postal Zip Code Number of collateralized agreements Number Of Collateralized Agreements Number Of Collateralized Agreements PIK interest income Investment income, interest, paid-in-kind Reversal of previously recorded PIK interest Interest Income, Operating, Paid in Kind Net expenses Investment Company, Expense, after Reduction of Fee Waiver and Reimbursement Total Square Feet Area of Real Estate Property Interest Income Interest income Interest Income, Operating Borrowings allowable per dollar of equity Debt Outstanding Allowable Per Dollar Of Equity Debt Outstanding Allowable Per Dollar Of Equity Preferred stock, shares outstanding (in shares) Preferred Stock, Shares Outstanding UniTek Global Services, Inc. UniTek Global Services, Inc. [Member] UniTek Global Services, Inc. Accumulated Net Investment Income Retained Earnings, Accumulated Net Investment Income (Loss) [Member] Retained Earnings, Accumulated Net Investment Income (Loss) Total investments Investments, Fair Value Disclosure Number of investments on non-accrual Investment Owned, Nonaccrual, Number Of Investments Investment Owned, Nonaccrual, Number Of Investments Information Technology Technology Sector [Member] Debt Instrument [Axis] Debt Instrument [Axis] Preferred equity dividend rate Preferred Stock, Dividend Rate, Percentage Distributions paid Payments of Ordinary Dividends, Common Stock Shares issued from the reinvestment of distributions (in shares) Stock Issued During Period, Shares, Dividend Reinvestment Plan $15,000 SBA-guaranteed debentures, Issued September 2018 USD 15,000 SBA Guaranteed Debentures, Issued September 2018 [Member] USD 15,000 SBA Guaranteed Debentures, Issued September 2018 Maximum borrowing capacity Line of Credit Facility, Maximum Borrowing Capacity Minimum asset coverage ratio Asset Coverage, Required Ratio, Minimum Asset Coverage, Required Ratio, Minimum PIK Investment, Interest Rate, Paid in Kind Foreside Financial Group, LLC Foreside Financial Group, LLC [Member] Foreside Financial Group, LLC Investment Company, Net Assets [Roll Forward] Investment Company, Net Assets [Roll Forward] CG Group Holdings, LLC CG Group Holdings, LLC [Member] CG Group Holdings, LLC Weighted average interest rate Long-Term Debt, Weighted Average Interest Rate, over Time Per share data: Investment Company, Per Share Data [Abstract] Investment Company, Per Share Data Investment Company, Nonconsolidated Subsidiary [Axis] Investment Company, Nonconsolidated Subsidiary [Axis] Management fee Management Fee Expense Paid in capital in excess of par Additional Paid in Capital Deferred tax liability Increase (Decrease) In Deferred Income Tax Liability Increase (Decrease) In Deferred Income Tax Liability Proceeds from sales and paydowns of investments Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales and Settlements Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Sales and Settlements Fair Value Measurement [Domain] Fair Value Measurement [Domain] Financial Instruments [Domain] Financial Instruments [Domain] Accumulated Net Realized (Losses) Gains Retained Earnings, Accumulated Net Realized Gain (Loss) [Member] Retained Earnings, Accumulated Net Realized Gain (Loss) Interest rate margin Debt Instrument, Basis Spread on Variable Rate ACI Group Holdings, Inc. ACI Group Holdings, Inc. [Member] ACI Group Holdings, Inc. Statistical Measurement [Domain] Statistical Measurement [Domain] Interest Rate Interest rate Investment Interest Rate DB Credit Facility DB Credit Facility [Member] DB Credit Facility Stated interest rate Debt Instrument, Interest Rate, Stated Percentage Valuation Approach and Technique [Axis] Valuation Approach and Technique [Axis] Number of portfolio companies Investment Company, Number Of Portfolio Companies Investment Company, Number Of Portfolio Companies $13,950 SBA-guaranteed debentures, Issued March 2016 USD 13,950 SBA Guaranteed Debentures, Issued March 2016 [Member] USD 13,950 SBA Guaranteed Debentures, Issued March 2016 SBA Annual Charge Debt Instrument, Annual Charge, Percentage Debt Instrument, Annual Charge, Percentage Investment, Issuer Name [Domain] Investment, Issuer Name [Domain] Non-cash investment income Other Noncash Income (Expense) Effect of foreign exchange rate changes on cash and cash equivalents Effect of Exchange Rate on Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Including Disposal Group and Discontinued Operations Income taxes paid Income Taxes Paid Common stock repurchased Treasury Stock, Value, Acquired, Cost Method QID TRH Holdings, LLC QID TRH Holdings, LLC [Member] QID TRH Holdings, LLC Variable Rate Period [Domain] Variable Rate Period [Domain] Variable Rate Period [Domain] Net increase (decrease) in net assets resulting from operations/ members capital Numerator for basic earnings per share Numerator for increase in net assets per share Net Income (Loss) Attributable to Parent Measurement Input Type [Domain] Measurement Input Type [Domain] 2018A Unsecured Notes Unsecured Notes Effective January 2018 [Member] Unsecured Notes Effective January 2018 Number of fee components Management And Service Fees, Number Of Components Management And Service Fees, Number Of Components Debt Investments Debt Securities [Member] Drawn revolving credit facilities Drawn Revolving Credit Facilities [Member] Drawn Revolving Credit Facilities Subsequent Event Type [Axis] Subsequent Event Type [Axis] Dividend Income Dividend income Dividend Income, Operating Net investment income before income taxes Net investment income before income taxes Net Investment Income NM YI LLC NM YI, LLC NM YI LLC [Member] NM YI LLC Equity Component [Domain] Equity Component [Domain] Subsequent Event Type [Domain] Subsequent Event Type [Domain] Indirect administrative expenses Investment Company, Fee Paid Indirectly Discount rate Measurement Input, Discount Rate [Member] Redemption as a percentage of principal Debt Instrument, Redemption Price, Percentage Increase in interest rate margin during an event of default Debt Instrument, Basis Spread On Variable Rate, Increase During Event Of Default Debt Instrument, Basis Spread On Variable Rate, Increase During Event Of Default Ownership [Axis] Ownership [Axis] Education Management II LLC Education Management II LLC [Member] Education Management II LLC Interest rate Related Party Transaction, Rate Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] Valuation Approach and Technique [Domain] Valuation Approach and Technique [Domain] Net change in unrealized appreciation (depreciation) Debt and Equity Securities, Unrealized Gain (Loss) [Member] Debt and Equity Securities, Unrealized Gain (Loss) Related Party [Domain] Related Party [Domain] Fair value Fair Value Measurement, Policy [Policy Text Block] After Reinvestment Period After Reinvestment Period [Member] After Reinvestment Period Net investment income (in dollars per share) Investment Company, Investment Income (Loss), Per Share Packaging Container and Packaging Sector [Member] Issuances of common stock (in shares) Stock Issued During Period, Shares, New Issues And Dividend Reinvestment Plan Stock Issued During Period, Shares, New Issues And Dividend Reinvestment Plan Investments at fair value Investments, At Fair Value [Member] Investments, At Fair Value Supplemental disclosure of cash flow information Supplemental Cash Flow Information [Abstract] Interest income (excluding Payment-in-kind ("PIK") interest income) Interest Income, Operating, Paid in Cash Total expenses, net of waivers/reimbursements Investment Company, Expense Ratio Including Fee Waiver And Reimbursements Investment Company, Expense Ratio Including Fee Waiver And Reimbursements Less: expenses waived and reimbursed (See Note 5) Fees waived by the administrator Investment Company, Voluntary Fee Waived Deca Dental Holdings LLC Deca Dental Holdings LLC [Member] Deca Dental Holdings LLC Interest Rate Type Interest Rate Type Risk [Member] Interest Rate Type Risk Statement of Cash Flows [Abstract] Number of investments held Investment Company, Number Of Investments Held Investment Company, Number Of Investments Held Credit facility Credit facility outstanding balance Long-Term Line of Credit Entity Address, State or Province Transfers out of Level III Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Transfers out of Level 3 Level I Fair Value, Inputs, Level 1 [Member] Adjustment for dilutive effect of Convertible Notes (in shares) Incremental Common Shares Attributable to Dilutive Effect of Conversion of Debt Securities Investment Company, Nonconsolidated Subsidiary [Table] Investment Company, Nonconsolidated Subsidiary [Table] Floating rates Debt Securities, Floating Interest Rate [Member] Debt Securities, Floating Interest Rate Payable to affiliates Increase (Decrease) in Due to Affiliates Receivable from unsettled securities sold Receivable, Investment, Sale Spread Investment, Basis Spread, Variable Rate Securities Financing Transaction [Table] Securities Financing Transaction [Table] Summary of Significant Accounting Policies Basis of Presentation and Significant Accounting Policies [Text Block] Adjustment for interest on Convertible Notes and incentive fees, net Dilutive Securities, Effect on Basic Earnings Per Share, Dilutive Convertible Securities Net cash flows used in operating activities Net Cash Provided by (Used in) Operating Activities GS Acquisitionco, Inc GS Acquisitionco, Inc [Member] GS Acquisitionco, Inc SkyKnight II SkyKnight Income II, LLC [Member] SkyKnight Income II, LLC Payment received for settlement Proceeds from Legal Settlements Related Party Transaction [Domain] Related Party Transaction [Domain] Specialtycare, Inc. Specialtycare, Inc. [Member] Specialtycare, Inc. Other liabilities Other Liabilities Management fees waived Less: management fee waiver Investment Company, Contractual Fee Waived, Management Fee Investment Company, Contractual Fee Waived, Management Fee Income tax expense Investment Income, Operating, Tax Expense (Benefit) Second lien Debt Securities, Second Lien [Member] Debt Securities, Second Lien Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table] Distributions declared to stockholders from net investment income (in dollars per share) Investment Company, Distribution to Shareholders, Per Share Other Commitments [Domain] Other Commitments [Domain] Trinity Air Consultants Holdings Corporation Trinity Air Consultants Holdings Corporation [Member] Trinity Air Consultants Holdings Corporation Legal Entity [Axis] Permian Trust Permian Trust [Member] Permian Trust Financial Highlights Investment Company, Financial Highlights [Text Block] Total liabilities Liabilities NMNLC Credit Facility II NMNLC Credit Facility II [Member] NMNLC Credit Facility II Geographical [Axis] Geographical [Axis] $19,000 SBA-guaranteed debentures, Issued September 2019 USD 19,000 SBA Guaranteed Debentures, Issued September 2019 [Member] USD 19,000 SBA Guaranteed Debentures, Issued September 2019 Net proceeds from shares sold Stock Issued During Period, Value, New Issues Accounting Changes and Error Corrections [Abstract] Industry Sector [Domain] Industry Sector [Domain] Receivable from affiliates Due from Affiliates PPV Intermediate Holdings LLC PPV Intermediate Holdings LLC [Member] PPV Intermediate Holdings LLC Recorded Future, Inc. Recorded Future, Inc. [Member] Recorded Future, Inc. Entity Common Stock, Shares Outstanding Fair Value Hierarchy and NAV [Domain] Fair Value Hierarchy and NAV [Domain] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Line Items] Revolving Credit Facility Revolving Credit Facility [Member] Schedule of Properties Property, Plant and Equipment [Table Text Block] Investment, Identifier [Axis] Investment, Identifier [Axis] DOCS, MSO, LLC DOCS, MSO, LLC [Member] DOCS, MSO, LLC Preferred stock, shares issued (in shares) Preferred Stock, Shares Issued Document Type Borrowings Long-Term Debt [Text Block] Concentration Risk Benchmark [Domain] Concentration Risk Benchmark [Domain] NM NL Holdings LP / NM GP Holdco LLC NM NL Holdings LP / NM GP Holdco LLC [Member] NM NL Holdings LP / NM GP Holdco LLC GC Waves Holdings, Inc. GC Waves Holdings, Inc. [Member] GC Waves Holdings, Inc. Schedule of Financial Highlights Investment Company, Financial Highlights [Table Text Block] Unfunded commitments on revolving credit facilities Unfunded Debt Securities, Revolving Credit Facilities [Member] Unfunded Debt Securities, Revolving Credit Facilities Common Class B Common Class B [Member] Entity Shell Company Deferred income tax provision Deferred Income Tax Expense (Benefit) Conversion premium Debt Instrument, Convertible Conversion Premium Debt Instrument, Convertible Conversion Premium (Increase) decrease in operating assets: Increase (Decrease) in Operating Assets [Abstract] Financial Instrument [Axis] Financial Instrument [Axis] Accrual for offering costs Stock Issuance Costs Incurred but Not Yet Paid Stock Issuance Costs Incurred but Not Yet Paid Total gains or losses included in earnings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Gain (Loss) Included in Earnings Adjustments to reconcile net increase in net assets resulting from operations to net cash (used in) provided by operating activities: Adjustments To Reconcile Net Assets From Operations To Cash Provided by (Used in) Operating Activities [Abstract] Adjustments To Reconcile Net Assets From Operations To Cash Provided by (Used in) Operating Activities Affiliated entity Affiliated Entity [Member] Subsequent Event Subsequent Event [Member] CentralSquare Technologies, LLC CentralSquare Technologies, LLC [Member] CentralSquare Technologies, LLC Purchase amount per $1,000 principal amount Debt Instrument, Repurchase Amount, Per USD1000 Of Principal Debt Instrument, Repurchase Amount, Per USD1000 Of Principal Document Period End Date American Achievement Corporation (aka AAC Holding Corp.) American Achievement Corporation (aka AAC Holding Corp.) [Member] American Achievement Corporation (aka AAC Holding Corp.) Offering costs Offering costs Adjustments to Additional Paid in Capital, Stock Issued, Issuance Costs Administration Agreement Administration Agreement [Member] Administration Agreement Eagle Infrastructure Super HoldCo, LLC Eagle Infrastructure Super HoldCo, LLC [Member] Eagle Infrastructure Super HoldCo, LLC NM JRA LLC NM JRA LLC [Member] NM JRA LLC Total assets Assets Debt Disclosure [Abstract] Earnings Per Share [Abstract] Proceeds from paydowns of investments Proceeds from Disposition of Investment, Operating Activity Income Statement Location [Axis] Income Statement Location [Axis] Specialty Chemicals & Materials Chemicals Sector [Member] Securities purchased under collateralized agreements to resell (cost of $30,000 and $30,000, respectively) Fair value of securities purchased under collateralized agreements to resell Securities Purchased under Agreements to Resell Unsecured Notes Unsecured Debt [Member] Members' capital Members' Equity Investment, Issuer Affiliation [Domain] Investment, Issuer Affiliation [Domain] Net cash flows (used in) provided by financing activities Net Cash Provided by (Used in) Financing Activities Borrowings Long-Term Debt, Fair Value Accounting Policies [Abstract] Stock repurchase program, authorized amount Stock Repurchase Program, Authorized Amount Interest expense Interest Expense, Debt Schedule of Principal Amounts and Fair Values of Borrowings Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table Text Block] Borrowings Long-Term Debt, Unclassified [Abstract] Credit Facility Line of Credit [Member] Other Commitments [Axis] Other Commitments [Axis] Net assets Stockholders' Equity Attributable to Parent [Abstract] Auctane, Inc. (fka Stamps.com) Auctane, Inc. (fka Stamps.com) [Member] Auctane, Inc. (fka Stamps.com) Interest and dividend receivable Dividend and Interest Receivable Earnings Per Share Earnings Per Share [Text Block] Payable to affiliates Affiliate borrowings outstanding Due to Affiliate Ansira Holdings, Inc. Ansira Holdings, Inc. [Member] Ansira Holdings, Inc. Schedule of Investments [Line Items] Schedule of Investments [Line Items] Sierra Hamilton Holdings Corporation Sierra Hamilton Holdings Corporation [Member] Sierra Hamilton Holdings Corporation Receivable from affiliates Increase (Decrease) Due from Affiliates Holdings Credit Facility, Broadly Syndicated Loans Holdings Credit Facility, Broadly Syndicated Loans [Member] Holdings Credit Facility, Broadly Syndicated Loans Cash and cash equivalents Cash and Cash Equivalents, Policy [Policy Text Block] Related Party Transaction [Line Items] Related Party Transaction [Line Items] Pre-Incentive Fee Net Investment Income Incentive fee, excluding accrued capital gains incentive fees Investment Management Agreement, Pre-Incentive Fee Net Investment Income [Member] Investment Management Agreement, Pre-Incentive Fee Net Investment Income Principal Amount, Par Value Principal amount or par value Principal amount Investment Owned, Balance, Principal Amount $51,000 SBA-guaranteed debentures, Issued September 2020 USD 51,000 SBA Guaranteed Debentures, Issued September 2020 [Member] USD 51,000 SBA Guaranteed Debentures, Issued September 2020 Entity Registrant Name Shares sold (in shares) Stock Issued During Period, Shares, New Issues Issuance discount percentage Guaranteed Debt, Issuance Discount Guaranteed Debt, Issuance Discount Purchase amount Debt Instrument, Repurchased Face Amount Investments in and Advances to Affiliates, at Fair Value [Roll Forward] Investments in and Advances to Affiliates, at Fair Value [Roll Forward] Tahoe Finco, LLC Tahoe Finco, LLC [Member] Tahoe Finco, LLC 2017A Unsecured Notes Unsecured Notes Effective June 2017 [Member] Unsecured Notes Effective June 2017 Edmentum Inc. Edmentum Inc. [Member] Edmentum Inc. Subsequent Events Subsequent Events [Text Block] KAMC Holdings, Inc KAMC Holdings, Inc [Member] KAMC Holdings, Inc Coupa Holdings, LLC Coupa Holdings, LLC [Member] Coupa Holdings, LLC Net realized gains on investments and foreign currency Net Realized Gains (Losses) Realized Gain (Loss), Investment, Derivative, and Foreign Currency Transaction Price Change, Operating, before Tax Regulatory capital Small Business Association Regulation, Regulatory Capital, Actual Small Business Association Regulation, Regulatory Capital, Actual Entity Address, City or Town Groundworks, LLC Groundworks, LLC [Member] Groundworks, LLC Expenses Operating Expenses [Abstract] Total net assets Beginning of the period End of the period Stockholders' Equity, Including Portion Attributable to Noncontrolling Interest Cash received for purchase of undrawn portion of revolving credit or delayed draw facilities Proceeds from Purchase of Undrawn Portion of Investment, Operating Activity Proceeds from Purchase of Undrawn Portion of Investment, Operating Activity Industry Sector [Axis] Industry Sector [Axis] Asset Class [Domain] Asset Class [Domain] CRCI Longhorn Holdings, Inc. CRCI Longhorn Holdings, Inc. [Member] CRCI Longhorn Holdings, Inc. Low Minimum Minimum [Member] Conversion rate Debt Instrument, Convertible, Conversion Ratio Non-controlling interest in New Mountain Net Lease Corporation Stockholders' Equity Attributable to Noncontrolling Interest Statement of Financial Position [Abstract] Entity Emerging Growth Company CFS Management, LLC CFS Management, LLC [Member] CFS Management, LLC Associations, Inc. Associations, Inc. [Member] Associations, Inc. Total net decrease in net assets resulting from capital transactions Investment Company, Capital Transactions, Increase (Decrease) Investment Company, Capital Transactions, Increase (Decrease) Common stock, par value (in dollars per share) Common Stock, Par or Stated Value Per Share Debt to equity ratio, maximum Debt Instrument, Covenant, Debt To Equity Ratio, Maximum Debt Instrument, Covenant, Debt To Equity Ratio, Maximum Appriss Health, LLC Appriss Health, LLC [Member] Appriss Health, LLC NMFC Senior Loan Program III LLC SLP III NMFC Senior Loan Program III LLC [Member] NMFC Senior Loan Program III LLC Issuances of common stock Stock Issued During Period, Value, New Issues And Dividend Reinvestment Plan Stock Issued During Period, Value, New Issues And Dividend Reinvestment Plan Commitments and Contingencies Disclosure [Abstract] Deferred financing costs (net of accumulated amortization of $49,187 and $47,531, respectively) Deferred financing costs (net of accumulated amortization) Debt Issuance Costs, Net Fair Value Fair Value Disclosures [Text Block] Unearned dividend income Investment Owned, Unearned Dividend Income Investment Owned, Unearned Dividend Income Gross Additions Investments in and Advances to Affiliates, at Fair Value, Gross Additions Trading Symbol Entity File Number DCA Investment Holding, LLC DCA Investment Holding, LLC [Member] DCA Investment Holding, LLC Market & income approach Valuation, Market And Income Approach [Member] Valuation, Market And Income Approach Schedule of Summarized Financial Information for Nonconsolidated Entities Investment Company, Nonconsolidated Subsidiary, Summarized Financial Information [Table Text Block] Canada CANADA Repayment of Unsecured Notes Repayment of Unsecured Notes Repayments of Unsecured Debt Administrative expenses Administrative Fees Expense Lender Cost Of Funds Rate Lender Cost Of Funds Rate [Member] Lender Cost Of Funds Rate Fair Value Property, Plant, and Equipment, Fair Value Disclosure Regulation Regulatory Capital Requirements For Investment Companies [Text Block] Regulatory Capital Requirements For Investment Companies Software Software Sector [Member] Software Sector Borrowing capacity, first percentage Line Of Credit Facility, Borrowing Capacity, First Percentage Line Of Credit Facility, Borrowing Capacity, First Percentage Concentration Risk Benchmark [Axis] Concentration Risk Benchmark [Axis] Wealth Enhancement Group, LLC Wealth Enhancement Group, LLC [Member] Wealth Enhancement Group, LLC Use of estimates Use of Estimates, Policy [Policy Text Block] Distributions Investment Company, Distribution [Policy Text Block] Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) / Eagle Infrastructure Super HoldCo, LLC Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.), Eagle Infrastructure Super HoldCo, LLC [Member] Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.), Eagle Infrastructure Super HoldCo, LLC Reduced base management fee percentage Management And Service Fees, Base Rate, Target Under Waiver Agreement Management And Service Fees, Base Rate, Target Under Waiver Agreement Investment extension period Investment Company, Nonconsolidated Subsidiary, Investment Extension Period Investment Company, Nonconsolidated Subsidiary, Investment Extension Period Investment Type Investment Type Concentration Risk [Member] Investment Type Concentration Risk Holdings Credit Facility Holdings Credit Facility [Member] Holdings Credit Facility LIBOR London Interbank Offered Rate (LIBOR) [Member] QID NGL, LLC QID NGL, LLC [Member] QID NGL, LLC Australia AUSTRALIA Prime Rate Prime Rate [Member] Subsequent Events [Abstract] Percent of Total Investments at Fair Value Concentration Risk, Percentage Annualized catch-up threshold Investment Management Agreement, Annualized Catch-Up Threshold [Member] Investment Management Agreement, Annualized Catch-Up Threshold Cash and other assets Cash And Other Assets Cash And Other Assets Variable Rate Component [Domain] Variable Rate Component [Domain] Variable Rate Component [Domain] Notorious Topco, LLC Notorious Topco, LLC [Member] Notorious Topco, LLC Credit Facility [Domain] Credit Facility [Domain] Increase (decrease) in operating liabilities: Increase (Decrease) in Operating Liabilities [Abstract] Non-cash dividend income Investment income, dividend, paid-in-kind Dividend Income, Operating, Paid in Kind Cash interest paid Interest Paid, Excluding Capitalized Interest, Operating Activities OA Buyer, Inc. OA Buyer, Inc. [Member] OA Buyer, Inc. Basic earnings per share (in dollars per share) Earnings Per Share, Basic Borrowing capacity, fourth percentage Line Of Credit Facility, Borrowing Capacity, Fourth Percentage Line Of Credit Facility, Borrowing Capacity, Fourth Percentage Management fee payable Management Fee Payable $4,000 SBA-guaranteed debentures, Issued September 2016 USD 4,000 SBA Guaranteed Debentures, Issued September 2016 [Member] USD 4,000 SBA Guaranteed Debentures, Issued September 2016 Kaseya Inc Kaseya Inc [Member] Kaseya Inc Distributions related to non-controlling interest in NMNLC Payments to Noncontrolling Interests Securities purchased under collateralized agreements to resell Net change in unrealized depreciation of securities purchased under collateralized agreements to resell Net change in unrealized depreciation of securities purchased under collateralized agreements to resell Unrealized Gain (Loss), Securities Repurchased Under Agreements To Resell Unrealized Gain (Loss), Securities Repurchased Under Agreements To Resell Gross Redemptions Investments in and Advances to Affiliates, at Fair Value, Gross Reductions Concentration Risk Type [Domain] Concentration Risk Type [Domain] Counterparty Name [Domain] Counterparty Name [Domain] Damages sought Loss Contingency, Damages Sought, Value Total net assets of New Mountain Finance Corporation Stockholders' Equity Attributable to Parent Consumer Services Consumer Services Sector [Member] Consumer Services Sector Percent of Net Assets Investment Owned, Percent of Net Assets Unearned interest income Investment Owned, Unearned Interest Income Investment Owned, Unearned Interest Income Total Investments [Member] Line of Credit Facility [Line Items] Line of Credit Facility [Line Items] Ownership [Domain] Ownership [Domain] Business Products Business Products Sector [Member] Business Products Sector Entity Interactive Data Current Sale of stock, number of shares issued (in shares) Sale of Stock, Number of Shares Issued in Transaction Property, Plant and Equipment [Table] Property, Plant and Equipment [Table] Dividend income Dividend Income, Operating, Paid in Cash Per share market value (in dollars per share) Investment Company, Market Value Per Share Increases in dividend in excess (in dollars per share) Debt Instrument, Convertible, Dividends In Excess, Increase Debt Instrument, Convertible, Dividends In Excess, Increase Holdings Credit Facility, Excluding Broadly Syndicated Loans Holdings Credit Facility, Excluding Broadly Syndicated Loans [Member] Holdings Credit Facility, Excluding Broadly Syndicated Loans Portion at Fair Value Measurement [Member] Portion at Fair Value Measurement [Member] Business Services Business Services Sector [Member] Business Services Sector ADG, LLC ADG, LLC [Member] ADG, LLC Net decrease in cash and cash equivalents Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents, Period Increase (Decrease), Excluding Exchange Rate Effect Capital transactions Investment Company, Capital Share Transactions [Abstract] Common Stock Common Stock [Member] Convey Health Solutions, Inc. Convey Health Solutions, Inc. [Member] Convey Health Solutions, Inc. Borrowing capacity, second percentage Line Of Credit Facility, Borrowing Capacity, Second Percentage Line Of Credit Facility, Borrowing Capacity, Second Percentage Financing commitment letter to purchase investments Financing Commitment Letter to Purchase Investments [Member] Financing Commitment Letter to Purchase Investments AmeriVet Partners Management, Inc. AmeriVet Partners Management, Inc. [Member] AmeriVet Partners Management, Inc. Controlled Investment, Affiliated Issuer, Controlled [Member] Class of Stock [Axis] Class of Stock [Axis] Portfolio turnover Investment Company, Portfolio Turnover Incomes taxes Income Tax, Policy [Policy Text Block] Revenue recognition Revenue [Policy Text Block] Safety Borrower Holdings LLC Safety Borrower Holdings LLC [Member] Safety Borrower Holdings LLC Statement [Table] Statement [Table] 2018B Unsecured Notes Unsecured Notes Effective July 2018 [Member] Unsecured Notes Effective July 2018 Interest Rate Debt Instrument, Interest Rate, Effective Percentage Total management fee Investment Company, Management Fee Expense, After Reduction Of Fee Waiver Investment Company, Management Fee Expense, After Reduction Of Fee Waiver Reinvestment of distributions Stock Issued During Period, Value, Dividend Reinvestment Plan Haven Midstream LLC / Haven Midstream Holdings LLC / QID TRH Holdings LLC Haven Midstream LLC And Haven Midstream Holdings LLC And QID TRH Holdings LLC [Member] Haven Midstream LLC And Haven Midstream Holdings LLC And QID TRH Holdings LLC Document Quarterly Report Investment Company, Changes in Net Assets [Line Items] Investment Company, Changes in Net Assets [Line Items] Pioneer Buyer I, LLC Pioneer Buyer I, LLC [Member] Pioneer Buyer I, LLC Net change in unrealized appreciation (depreciation): Investment Owned, Recognized Unrealized Appreciation (Depreciation), Net [Abstract] Investment Owned, Recognized Unrealized Appreciation (Depreciation), Net 2018 Convertible Notes 2018 Convertible Notes [Member] 2018 Convertible Notes Schedule of Fair Value Measurement Inputs and Valuation Techniques Fair Value Measurement Inputs and Valuation Techniques [Table Text Block] Hong Kong HONG KONG Statistical Measurement [Axis] Statistical Measurement [Axis] Total return based on market value Investment Company, Total Return, Market Value Related Party Transaction [Axis] Related Party Transaction [Axis] NM DRVT LLC NM DRVT LLC [Member] NM DRVT LLC Accumulated undistributed earnings Retained Earnings (Accumulated Deficit) Transfers into Level III Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Transfers Into Level 3 Cash equivalents Cash Equivalents, at Carrying Value Proceeds from Convertible Debt Proceeds from Convertible Debt Provision for taxes Provision for taxes Realized and Unrealized Gain (Loss), Investment and Derivative, Operating, Tax Expense (Benefit) Equity Components [Axis] Equity Components [Axis] Distributions declared to stockholders from net investment income Distributions declared Investment Company, Dividend Distribution Unrealized appreciation (depreciation) for the period relating to those Level III assets that were still held by the Company at the end of the period: Fair Value, Asset, Recurring Basis, Still Held, Unrealized Gain (Loss) Litigation Case [Domain] Litigation Case [Domain] Agreements Related Parties Related Party Transactions Disclosure [Text Block] Investments Investment Holdings, Schedule of Investments [Text Block] Schedule of Investments [Abstract] Maximum purchase amount Debt Instrument, Repurchase Face Amount, Maximum Debt Instrument, Repurchase Face Amount, Maximum Investment Company, Nonconsolidated Subsidiary [Domain] Investment Company, Nonconsolidated Subsidiary [Domain] Document Fiscal Year Focus Community Brands ParentCo, LLC Community Brands ParentCo, LLC [Member] Community Brands ParentCo, LLC IMO Investor Holdings, Inc. IMO Investor Holdings, Inc. [Member] IMO Investor Holdings, Inc. United Kingdom UNITED KINGDOM Cash flows from operating activities Net Cash Provided by (Used in) Operating Activities [Abstract] Statement [Line Items] Statement [Line Items] EBITDA multiple Measurement Input, EBITDA Multiple [Member] Fair Value Measurement Inputs and Valuation Techniques [Table] Fair Value Measurement Inputs and Valuation Techniques [Table] Payable for unsettled securities purchased Payable, Investment, Purchase Deferred offering costs Deferred Charges, Policy [Policy Text Block] Investments Net change in unrealized (appreciation) depreciation of investments Net change in unrealized appreciation (depreciation) of investments Debt and Equity Securities, Unrealized Gain (Loss) Investments at fair value Fair Value Fair Value, Beginning Balance Fair Value, Ending Balance Fair value Investment Owned, at Fair Value Quarterly catch-up threshold Investment Management Agreement, Quarterly Catch-Up Threshold [Member] Investment Management Agreement, Quarterly Catch-Up Threshold Sun Acquirer Corp. Sun Acquirer Corp. [Member] Sun Acquirer Corp. Variable Rate [Domain] Variable Rate [Domain] Black Elk Trustee Black Elk Trustee [Member] Black Elk Trustee Fair Value Hierarchy and NAV [Axis] Fair Value Hierarchy and NAV [Axis] Granicus, Inc. Granicus, Inc. [Member] Granicus, Inc. SBA-guaranteed debentures SBA-guaranteed debentures outstanding Guaranteed Debt Guaranteed Debt Recent Accounting Standards Updates Accounting Standards Update and Change in Accounting Principle [Text Block] Investment, Issuer Name [Axis] Investment, Issuer Name [Axis] Current income tax expense Current Income Tax Expense (Benefit) Interest and dividend receivable Increase (Decrease) in Interest and Dividends Receivable $28,795 SBA-guaranteed Debentures, Issued September 2015 USD 28,795 SBA Guaranteed Debentures, Issued September 2015 [Member] USD 28,795 SBA Guaranteed Debentures, Issued September 2015 Document Transition Report Increase (decrease) in net assets resulting from operations: Investment Company, Net Assets from Operations, Increase (Decrease) [Abstract] Local Phone Number Recent accounting standards updates New Accounting Pronouncements, Policy [Policy Text Block] Available borrowing capacity Line of Credit Facility, Current Borrowing Capacity Consumer Products Consumer Products Sector [Member] Consumer Products Sector Investment Management Agreement Investment Management Agreement [Member] Investment Management Agreement YLG Holdings, Inc. YLG Holdings, Inc. [Member] YLG Holdings, Inc. Number of shares outstanding (in shares) Common stock, shares outstanding (in shares) Shares outstanding at end of period (in shares) Common Stock, Shares, Outstanding National HME, Inc./NHME Holdings Corp. National HME, Inc. And NHME Holdings Corp. [Member] National HME, Inc. And NHME Holdings Corp. Idera, Inc. Idera, Inc. [Member] Idera, Inc. Unsecured Management Company Revolver Unsecured Management Company Revolver [Member] Unsecured Management Company Revolver Investment, Issuer Affiliation [Axis] Investment, Issuer Affiliation [Axis] Income tax provision Income Tax Expense (Benefit) Geographical [Domain] Geographical [Domain] Deferred tax liability Deferred Income Tax Liabilities, Net Calabrio, Inc. Calabrio, Inc. [Member] Calabrio, Inc. Preferred stock, par value (in dollars per share) Preferred Stock, Par or Stated Value Per Share Income Statement [Abstract] Paid in Capital in Excess of Par Additional Paid-in Capital [Member] Unfunded commitments on bridge facilities Bridge financing commitments Unfunded Debt Securities, Bridge Facilities [Member] Unfunded Debt Securities, Bridge Facilities Document Fiscal Period Focus Earnings per share—basic Earnings Per Share, Basic [Abstract] Xactly Corporation Xactly Corporation [Member] Xactly Corporation Maximum secured debt ratio Debt Instrument, Covenant, Secured Debt Ratio, Maximum Debt Instrument, Covenant, Secured Debt Ratio, Maximum Revenue multiple Measurement Input, Revenue Multiple [Member] Weighted average shares of common stock outstanding - diluted (in shares) Denominator for diluted weighted average share (in shares) Weighted Average Number of Shares Outstanding, Diluted Non-cash operating activities: Non Cash Operating And Investing Activities [Abstract] Non Cash Operating And Investing Activities NM NL Holdings, L.P. NM NL Holdings, L.P. [Member] NM NL Holdings, L.P. National HME, Inc. National HME, Inc. [Member] National HME, Inc. Coyote Buyer, LLC Coyote Buyer, LLC [Member] Coyote Buyer, LLC KPSKY Acquisition Inc. KPSKY Acquisition Inc. [Member] KPSKY Acquisition Inc. Other general and administrative expenses Other General and Administrative Expense USRP Holdings, Inc. USRP Holdings, Inc. [Member] USRP Holdings, Inc. Distributions paid per share (in dollars per share) Common Stock, Dividends, Per Share, Cash Paid Base Rate Base Rate [Member] Symplr Software Intermediate Holdings, Inc. Symplr Software Intermediate Holdings, Inc. [Member] Symplr Software Intermediate Holdings, Inc. Common stock, par value $0.01 per share, 200,000,000 shares authorized, and 100,937,026 and 100,937,026 shares issued and outstanding, respectively Common Stock, Value, Issued Principal Amount Reported Value Measurement [Member] Interest and other financing expenses Interest Expense, Policy [Policy Text Block] Debt Instrument, Name [Domain] Debt Instrument, Name [Domain] Apptio, Inc. Apptio, Inc. [Member] Apptio, Inc. Repurchase amount Investment Owned, Balance, Repurchase Amount Investment Owned, Balance, Repurchase Amount Other commitments Other Commitment Conversion price (in dollars per share) Debt Instrument, Convertible, Conversion Price Net Assets Investment Company, Net Assets [Text Block] Investment Company, Net Assets HS Purchaser, LLC / Help/Systems Holdings, Inc. HS Purchaser, LLC / Help/Systems Holdings, Inc. [Member] HS Purchaser, LLC / Help/Systems Holdings, Inc. Investment Company, Financial Highlights [Line Items] Investment Company, Financial Highlights [Line Items] Convertible Notes Convertible Debt [Member] SOFR Secured Overnight Financing Rate (SOFR) [Member] Secured Overnight Financing Rate (SOFR) Investment period Investment Company, Nonconsolidated Subsidiary, Investment Period Investment Company, Nonconsolidated Subsidiary, Investment Period Unfunded Debt Investments Unfunded Debt Securities [Member] Unfunded Debt Securities $24,000 SBA-guaranteed debentures, Issued March 2020 USD 24,000 SBA Guaranteed Debentures, Issued March 2020 [Member] USD 24,000 SBA Guaranteed Debentures, Issued March 2020 Other income Other Income Reversal of previously recorded other income Other Income Preferred stock, shares authorized (in shares) Preferred Stock, Shares Authorized Cash and cash equivalents at the beginning of the period Cash and cash equivalents at the end of the period Cash, Cash Equivalents, Restricted Cash, and Restricted Cash Equivalents Fixed rates Debt Securities, Fixed Interest Rate [Member] Debt Securities, Fixed Interest Rate Principal Amount, Shares (in shares) Principal amount (in shares) Investment Owned, Balance, Shares New Permian Holdco, Inc. / New Permian Holdco, L.L.C. New Permian Holdco, Inc. And New Permian Holdco, L.L.C. [Member] New Permian Holdco, Inc. And New Permian Holdco, L.L.C. Less: Net increase in net assets resulting from operations related to non-controlling interest in New Mountain Net Lease Corporation Net Income (Loss) Attributable to Noncontrolling Interest Commitments and Contingencies Commitments and Contingencies Disclosure [Text Block] Bluefin Holding, LLC Bluefin Holding, LLC [Member] Bluefin Holding, LLC Common stock, shares issued (in shares) Beginning of the period (in shares) End of the period (in shares) Common Stock, Shares, Issued Variable Rate Period [Axis] Variable Rate Period [Axis] Variable Rate Period New Mountain Finance Corporation Total Net Assets of NMFC Parent [Member] Income Statement Location [Domain] Income Statement Location [Domain] SBA-guaranteed debentures Guaranteed Debt [Member] Guaranteed Debt Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items] SBIC II New Mountain Finance SBIC II, L.P. [Member] New Mountain Finance SBIC II, L.P. Funded Investments Funded Securities [Member] Funded Securities OEConnection LLC OEConnection LLC [Member] OEConnection LLC Amendment Flag Professional fees Professional Fees Face amount Debenture Amount Debt Instrument, Face Amount Investment Type [Axis] Investment Type [Axis] Investments Investment, Policy [Policy Text Block] Proceeds from Credit Facility Proceeds from Long-Term Lines of Credit Litigation Case [Axis] Litigation Case [Axis] New Permian Holdco, L.L.C. New Permian Holdco, L.L.C. [Member] New Permian Holdco, L.L.C. Entity Current Reporting Status Funded Debt Investments Funded Debt Securities [Member] Funded Debt Securities Sale of stock, remaining amount available for issuance and sale Sale Of Stock, Remaining Stock Available For Issuance, Amount Sale Of Stock, Remaining Stock Available For Issuance, Amount Convertible Notes Convertible Debt Total return based on net asset value Investment Company, Total Return Total net increase (in dollars per share) Investment Company, Investment Income (Loss) from Operations, Per Share TVG-Edmentum Holdings, LLC / Edmentum Ultimate Holdings, LLC TVG-Edmentum Holdings, LLC And Edmentum Ultimate Holdings, LLC [Member] TVG-Edmentum Holdings, LLC And Edmentum Ultimate Holdings, LLC Jersey JERSEY Commitment fee percentage on unused capacity Line of Credit Facility, Unused Capacity, Commitment Fee Percentage Management fee payable Increase (Decrease) in Management Fee Payable Agent fee percentage Line Of Credit Facility, Agent Fee, Percentage Line Of Credit Facility, Agent Fee, Percentage Integro Parent Inc. Integro Parent Inc. [Member] Integro Parent Inc. First lien Debt Securities, First Lien [Member] Debt Securities, First Lien Counterparty Name [Axis] Counterparty Name [Axis] Schedule of Management Fees and Incentive Fees Schedule of Related Party Transactions [Table Text Block] Pre-Incentive Fee Net Investment Income exceeds catch-up threshold Investment Management Agreement, Pre-Incentive Fee Net Investment Income Exceeds Catch-Up Threshold [Member] Investment Management Agreement, Pre-Incentive Fee Net Investment Income Exceeds Catch-Up Threshold Infogain Corporation Infogain Corporation [Member] Infogain Corporation Preferred equity variable rate margin Preferred Stock, Dividend Rate, Basis Spread On Variable Rate Preferred Stock, Dividend Rate, Basis Spread On Variable Rate Investment Company, Nonconsolidated Subsidiary [Line Items] Investment Company, Nonconsolidated Subsidiary [Line Items] Non-controlled/Affiliated Investment, Affiliated Issuer, Noncontrolled [Member] Entity Incorporation, State or Country Code Cash paid for purchase of investments Payments for Purchase of Investment, Operating Activity Basis of accounting Basis of Accounting, Policy [Policy Text Block] Liabilities Liabilities [Abstract] Avalara, Inc. Avalara, Inc. [Member] Avalara, Inc. $41,000 SBA-guaranteed debentures, Issued March 2020 USD 41,000 SBA Guaranteed Debentures, Issued March 2020 [Member] USD 41,000 SBA Guaranteed Debentures, Issued March 2020 Daxko Acquisition Corporation Daxko Acquisition Corporation [Member] Daxko Acquisition Corporation SBIC I New Mountain Finance SBIC, L.P. [Member] New Mountain Finance SBIC, L.P. Unfunded commitments on delayed draws or other future funding commitments Other future funding commitments Unfunded Debt Securities, Delayed Draws Or Other Future Funding Commitments [Member] Unfunded Debt Securities, Delayed Draws Or Other Future Funding Commitments Numerator for diluted earnings per share: Net Income (Loss) Attributable to Parent, Diluted Average debt outstanding Long-Term Debt, Average Amount Outstanding Net Lease Real Estate Sector [Member] Net borrowings Long-Term Debt Sale of stock, price per share (in dollars per share) Sale of Stock, Price Per Share Net realized and unrealized gains Realized and Unrealized Gain (Loss), Investment, Derivative, and Foreign Currency Transaction, Operating, after Tax Common stock, shares authorized (in shares) Common Stock, Shares Authorized Schedule of Related Party Transactions, by Related Party [Table] Schedule of Related Party Transactions, by Related Party [Table] Unfunded Investments Unfunded Securities [Member] Unfunded Securities Pre-Incentive Fee Net Investment Income below catch-up threshold Investment Management Agreement, Pre-Incentive Fee Net Investment Income Below Catch-Up Threshold [Member] Investment Management Agreement, Pre-Incentive Fee Net Investment Income Below Catch-Up Threshold Shares redeemed for a promissory note (in shares) Investment Owned, Shares Redeemed Investment Owned, Shares Redeemed 2022A Unsecured Notes Unsecured Notes Effective June 2022 [Member] Unsecured Notes Effective June 2022 High Maximum Maximum [Member] Reimbursements to administrators as a percentage of gross assets Investment Company, Fee Reimbursement To Gross Assets Investment Company, Fee Reimbursement To Gross Assets Less: management fee waived (See Note 5) Investment Company, Contractual Fee Waived Quarterly hurdle rate Investment Management Agreement, Quarterly Hurdle Rate [Member] Investment Management Agreement, Quarterly Hurdle Rate Cash and cash equivalents Cash and Cash Equivalents, at Carrying Value Investments including delayed draw facilities Investments Including Delayed Draw Facilities [Member] Investments Including Delayed Draw Facilities Net realized gains (losses) on investments Debt and Equity Securities, Realized Gain (Loss) [Member] Debt and Equity Securities, Realized Gain (Loss) Agreement term Related Party Transaction, Term Related Party Transaction, Term New Benevis Topco, LLC New Benevis Topco, LLC [Member] New Benevis Topco, LLC Entity Small Business Measurement Basis [Axis] Measurement Basis [Axis] Deferred financing costs, accumulated amortization Accumulated Amortization, Debt Issuance Costs Other Commitments [Table] Other Commitments [Table] NM GP Holdco, LLC NM GP Holdco, LLC [Member] NM GP Holdco, LLC Accrual for deferred financing costs Debt Issuance Costs Incurred but Not Yet Paid Debt Issuance Costs Incurred but Not Yet Paid Distributions declared per share (in dollars per share) Common Stock, Dividends, Per Share, Declared Investment income Revenues [Abstract] Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table] Fair Value Disclosure of Asset and Liability Not Measured at Fair Value [Table] Brave Parent Holdings, Inc. Brave Parent Holdings, Inc. [Member] Brave Parent Holdings, Inc. Stock repurchase program Stockholders' Equity, Policy [Policy Text Block] Line of Credit Facility [Table] Line of Credit Facility [Table] Schedule of Portfolio Investments by Level in the Fair Value Hierarchy Fair Value, Assets Measured on Recurring Basis [Table Text Block] Foreign currency Net realized gains on translation of assets and liabilities in foreign currencies Foreign Currency Transaction Gain (Loss), Realized Net realized and unrealized (losses) gains (in dollars per share) Investment Company, Gain (Loss) on Investment, Per Share Regular distribution Regular Distribution [Member] Regular Distribution Amortization of premium on Convertible Notes Amortization of premium Amortization of Debt Discount (Premium) NM APP US LLC NM APP US LLC [Member] NM APP US LLC New Benevis Topco, LLC / New Benevis Holdco, Inc. New Benevis Topco, LLC And New Benevis Holdco, Inc. [Member] New Benevis Topco, LLC And New Benevis Holdco, Inc. Subordinated Subordinated Debt [Member] Title of 12(b) Security Formation and Business Purpose Organization, Consolidation and Presentation of Financial Statements Disclosure [Text Block] Investments threshold period past due for nonaccrual status Investment Owned, Nonaccrual, Threshold Period Past Due, Investment Owned, Nonaccrual, Threshold Period Past Due, Ordinary shares dividend rate Common Stock, Dividend Rate, Percentage Common Stock, Dividend Rate, Percentage Industry Type Industry Concentration Risk [Member] Industry Concentration Risk Unsecured Notes Unsecured Notes outstanding balance Unsecured Debt TMK Hawk Parent, Corp. TMK Hawk Parent, Corp. [Member] TMK Hawk Parent, Corp. Securities purchased under collateralized agreements to resell, cost Cost basis of securities purchased under collateralized agreements to resell Securities Purchased Under Agreements To Resell, Cost Securities Purchased Under Agreements To Resell, Cost Borrowing capacity, third percentage Line Of Credit Facility, Borrowing Capacity, Third Percentage Line Of Credit Facility, Borrowing Capacity, Third Percentage Bullhorn, Inc. Bullhorn, Inc. [Member] Bullhorn, Inc. Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) [Member] Eagle Infrastructure Services, LLC (fka FR Arsenal Holdings II Corp.) Subsequent Event [Line Items] Subsequent Event [Line Items] Collateralized agreements or repurchase financings Repurchase Agreements, Collateral, Policy [Policy Text Block] Debt Instrument [Line Items] Debt Instrument [Line Items] Interest payable Interest Payable Fair Value Measurement Inputs and Valuation Techniques [Line Items] Fair Value Measurement Inputs and Valuation Techniques [Line Items] Deferred financing costs paid Payments of Debt Issuance Costs Amortization of purchase discount Accretion (Amortization) of Discounts and Premiums, Investments Other Other Valuation Technique [Member] Other Valuation Technique Healthcare Healthcare Sector [Member] Haven Midstream Holdings LLC Haven Midstream Holdings LLC [Member] Haven Midstream Holdings LLC Netherlands NETHERLANDS Net increase in net assets Stockholders' Equity, Period Increase (Decrease) Fair Value Estimate of Fair Value Measurement [Member] Redemption of shares paid-in-kind Investment Company, Redemption of Shares, Paid-In-Kind Investment Company, Redemption of Shares, Paid-In-Kind Fair Value, Recurring and Nonrecurring [Table] Fair Value, Recurring and Nonrecurring [Table] Settlement amount awarded to other party Litigation Settlement, Amount Awarded to Other Party Foundational Education Group, Inc. Foundational Education Group, Inc. [Member] Foundational Education Group, Inc. Other expenses Other Expenses Sale of stock, consideration received Sale of Stock, Consideration Received on Transaction Sale of stock, issuance amount Sale Of Stock, Maximum Issuance, Amount Sale Of Stock, Maximum Issuance, Amount Other assets Increase (Decrease) in Other Operating Assets Exercise price adjustment percentage Debt Instrument, Convertible, Threshold Percentage of Stock Price Trigger Incentive fee percentage Management and Service Fees, Incentive Rate Net Unrealized Appreciation (Depreciation) Retained Earnings, Net Unrealized Appreciation (Depreciation) [Member] Retained Earnings, Net Unrealized Appreciation (Depreciation) Warrants Warrant [Member] Entity Filer Category Net proceeds from shares sold Net proceeds from offerings of common stock Proceeds from Issuance of Common Stock Weighted average shares of common stock outstanding - basic (in shares) Denominator for basic weighted average share (in shares) Average weighted shares outstanding for the period - basic (in shares) Weighted Average Number of Shares Outstanding, Basic 2019A Unsecured Notes Unsecured Notes Effective April 2019 [Member] Unsecured Notes Effective April 2019 AAH Topco, LLC AAH Topco, LLC [Member] AAH Topco, LLC United States UNITED STATES Financial Services Financial Services Sector [Member] $15,255 SBA-guaranteed debentures, Issued March 2018 USD 15,255 SBA Guaranteed Debentures, Issued March 2018 [Member] USD 15,255 SBA Guaranteed Debentures, Issued March 2018 Securities Financing Transaction [Line Items] Securities Financing Transaction [Line Items] Commitments and contingencies (See Note 9) Commitments and Contingencies Schedule of Investments Investment Holdings, Schedule of Investments [Table Text Block] Security Exchange Name Energy Energy Sector [Member] Value of shares reissued from repurchase program in connection with the distribution reinvestment plan Stock Issued Preferred stock, par value $0.01 per share, 2,000,000 shares authorized, none issued Preferred Stock, Value, Issued Interest payable Increase (Decrease) in Interest Payable, Net NM APP CANADA CORP NM APP Canada, Corp [Member] NM APP Canada, Corp Property, Plant and Equipment [Line Items] Property, Plant and Equipment [Line Items] Incentive fee payable Increase (Decrease) in Incentive Fee Payable Changes in Level III Portfolio Investments Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation [Table Text Block] Dividends [Domain] Dividends [Domain] Schedule of Long-Term Debt Instruments [Table] Schedule of Long-Term Debt Instruments [Table] Payable for unsettled securities purchased Increase (Decrease) in Payable for Investment Purchased Level III Fair Value, Inputs, Level 3 [Member] Oranje Holdco, Inc. Oranje Holdco, Inc. [Member] Oranje Holdco, Inc. Investment Company, Financial Highlights [Roll Forward] Investment Company, Financial Highlights [Roll Forward] Other assets Other Assets Education Education Sector [Member] Education Sector Incentive fee payable Incentive Fee Payable Investments [Domain] Investments [Domain] Non-Controlling Interest in NMNLC Noncontrolling Interest [Member] Base management fee percentage Management and Service Fees, Base Rate Distribution payable Distribution Payable Total liabilities and net assets Liabilities and Equity Knockout Intermediate Holdings I Inc. Knockout Intermediate Holdings I Inc. [Member] Knockout Intermediate Holdings I Inc. Term of debt Debt Instrument, Term Foreign currency Net change in unrealized (appreciation) depreciation on translation of assets and liabilities in foreign currencies Foreign Currency Transaction Gain (Loss), Unrealized Net realized gains (losses): Net Realized Gains (Losses) [Abstract] Net Realized Gains (Losses) Non-cash activity on investments Noncash Activity On Investments Noncash Activity On Investments Amortization of deferred financing costs Amortization of financing costs Amortization of Debt Issuance Costs Related Party [Axis] Related Party [Axis] Long-Term Debt, Type [Axis] Long-Term Debt, Type [Axis] Repayments of Credit Facility Repayments of Long-Term Lines of Credit Annualized hurdle rate Investment Management Agreement, Annualized Hurdle Rate [Member] Investment Management Agreement, Annualized Hurdle Rate Earnings per share—diluted Earnings Per Share, Diluted [Abstract] Organization, Consolidation and Presentation of Financial Statements [Abstract] Ownership percentage Limited Liability Company (LLC) or Limited Partnership (LP), Managing Member or General Partner, Ownership Interest Non-controlled/Non-affiliated Investment, Unaffiliated Issuer [Member] Non-cash financing activities: Noncash Financing Items [Abstract] Noncash Financing Items Level II Fair Value, Inputs, Level 2 [Member] Dividend reinvestment plan, share price percentage, threshold Dividend Reinvestment Plan, Share Price Percentage, Threshold Dividend Reinvestment Plan, Share Price Percentage, Threshold Deferred financing costs Debt, Policy [Policy Text Block] Current Fiscal Year End Date Variable Rate Component [Axis] Variable Rate Component [Axis] Variable Rate Component Supplemental distribution Supplemental Distribution [Member] Supplemental Distribution Total expenses, before waivers/reimbursements Investment Company, Expense Ratio Before Fee Waiver And Reimbursements Investment Company, Expense Ratio Before Fee Waiver And Reimbursements Purchases, including capitalized PIK and revolver fundings Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset, Purchases Diluted earnings per share (in dollars per share) Earnings Per Share, Diluted Sale of Stock [Domain] Sale of Stock [Domain] $37,500 SBA-guaranteed Debentures, Issued March 2015 USD 37,500 SBA Guaranteed Debentures, Issued March 2015 [Member] USD 37,500 SBA Guaranteed Debentures, Issued March 2015 NM GLCR LP NM GLCR LP [Member] NM GLCR LP Offering costs paid Offering expenses Payments of Stock Issuance Costs Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Fair Value, Assets Measured on Recurring Basis, Unobservable Input Reconciliation, Calculation [Roll Forward] Investments Net realized gains on investments Net realized losses on investments Debt and Equity Securities, Realized Gain (Loss) Investments in Majority-owned Subsidiaries Investments in Majority-owned Subsidiaries [Member] Concentration Risk Type [Axis] Concentration Risk Type [Axis] 2022 Convertible Notes 2022 Convertible Notes [Member] 2022 Convertible Notes Asset coverage ratio Investment Company, Senior Security, Indebtedness, Asset Coverage Ratio NM KRLN LLC NM KRLN LLC [Member] NM KRLN LLC Realized capital gains Accrued capital gains incentive fees Investment Management Agreement , Realized Capital Gains [Member] Investment Management Agreement , Realized Capital Gains Net asset value per share of New Mountain Finance Corporation (in dollars per share) Net asset value, beginning balance (in dollars per share) Net asset value, ending balance (in dollars per share) Net Asset Value Per Share Equity Equity Securities [Member] ATM Offering At the Market Offering [Member] At the Market Offering Investments excluding drawn revolvers Investments Excluding Drawn Revolvers [Member] Investments Excluding Drawn Revolvers Net investment income Investment Company, Investment Income (Loss) Ratio MRI Software LLC MRI Software LLC [Member] MRI Software LLC NMFC Senior Loan Program IV LLC SLP IV NMFC Senior Loan Program IV LLC [Member] NMFC Senior Loan Program IV LLC FS WhiteWater Borrower, LLC FS WhiteWater Borrower, LLC [Member] FS WhiteWater Borrower, LLC Total expenses Investment Income, Investment Expense Cardinal Parent, Inc. Cardinal Parent, Inc. [Member] Cardinal Parent, Inc. VT Topco, Inc. VT Topco, Inc. [Member] VT Topco, Inc. Upfront fees percentage Guaranteed Debt, Upfront Fees Guaranteed Debt, Upfront Fees Entity Address, Address Line One Net increase in shares outstanding (in shares) Stock Issued During Period, Shares, Period Increase (Decrease) $13,000 SBA-guaranteed debentures, Issued September 2017 USD 13,000 SBA Guaranteed Debentures, Issued September 2017 [Member] USD 13,000 SBA Guaranteed Debentures, Issued September 2017 Distribution & Logistics Transportation Sector [Member] Class of Stock [Domain] Class of Stock [Domain] IG Investments Holdings, LLC IG Investments Holdings, LLC [Member] IG Investments Holdings, LLC Fair Value Disclosures [Abstract] Pye-Barker Fire & Safety, LLC Pye-Barker Fire & Safety, LLC [Member] Pye-Barker Fire & Safety, LLC Maximum amount of borrowings available Line of Credit Facility, Remaining Borrowing Capacity Interest and other financing expenses Interest Expense Incentive fee Incentive Fee Expense Credit Facility [Axis] Credit Facility [Axis] New Benevis Holdco, Inc. New Benevis Holdco, Inc. [Member] New Benevis Holdco, Inc. NMC Crimson Holdings, Inc. NMC Crimson Holdings, Inc. [Member] NMC Crimson Holdings, Inc. Capital share activity Investment Company, Capital Share Transactions, Stock Issued [Abstract] Total investment income Gross Investment Income, Operating Investment Company, Financial Highlights [Table] Investment Company, Financial Highlights [Table] During Reinvestment Period During Reinvestment Period [Member] During Reinvestment Period Schedule of Information Related to Borrowings Schedule of Long-Term Debt Instruments [Table Text Block] Equity [Abstract] Subsequent Event [Table] Subsequent Event [Table] Investment Company [Abstract] Entity Tax Identification Number Schedule of Investments [Table] Schedule of Investments [Table] Education Management Corporation Education Management Corporation [Member] Education Management Corporation Net investment income Net investment income Investment Income, Operating, after Expense and Tax NMFC Credit Facility NMFC Credit Facility [Member] NMFC Credit Facility Equity and other Other Security Investments [Member] Tennessee Bidco Limited Tennessee Bidco Limited [Member] Tennessee Bidco Limited Fortis Solutions Group, LLC Fortis Solutions Group, LLC [Member] Fortis Solutions Group, LLC Investment Company, Changes in Net Assets [Table] Investment Company, Changes in Net Assets [Table] TigerConnect, Inc. TigerConnect, Inc. [Member] TigerConnect, Inc. Measurement Input Investment Owned, Fair Value, Measurement Input Investment Owned, Fair Value, Measurement Input Net change in unrealized appreciation (depreciation) of investments and foreign currency Net Change In Unrealized Appreciation (Depreciation) Unrealized Gain (Loss), Investment, Derivative, and Foreign Currency Transaction Price Change, Operating, before Tax Effective interest rate Debt Instrument, Interest Rate During Period Investment Funds (includes investments in joint ventures) Investment Funds, Including Investments In Joint Ventures [Member] Investment Funds, Including Investments In Joint Ventures $37,500 SBA-guaranteed Debentures, Issued September 2015 USD 37,500 SBA Guaranteed Debentures, Issued September 2015 [Member] USD 37,500 SBA Guaranteed Debentures, Issued September 2015 Non-usage fee Line of Credit Facility, Commitment Fee Amount Investments, cost Cost Cost basis Investment Owned, at Cost SkyKnight Alpha SkyKnight Income Alpha, LLC [Member] SkyKnight Income Alpha, LLC Entity Central Index Key LIBOR, SONIA or EURIBOR London Interbank Offered Rate (LIBOR) Or Sterling Overnight Interbank Average Rate (SONIA) Or Euro Interbank Offered Rate (EURIBOR) [Member] London Interbank Offered Rate (LIBOR) Or Sterling Overnight Interbank Average Rate (SONIA) Or Euro Interbank Offered Rate (EURIBOR) Measurement Input Type [Axis] Measurement Input Type [Axis] Entity [Domain] Strain On Banking System [Member] Strain On Banking System Number of incentive fee components Management And Service Fees, Incentive Rate, Number Of Components Management And Service Fees, Incentive Rate, Number Of Components City Area Code 2021A Unsecured Notes Unsecured Notes Effective January 2021 [Member] Unsecured Notes Effective January 2021 Assets Assets [Abstract] Average net assets for the period Average Net Assets Average Net Assets Long-Term Debt, Type [Domain] Long-Term Debt, Type [Domain] Syndigo LLC Syndigo LLC [Member] Syndigo LLC iCIMS, Inc. iCIMS, Inc. [Member] iCIMS, Inc. Other Commitments [Line Items] Other Commitments [Line Items] Statement of Stockholders' Equity [Abstract] Sale of Stock [Axis] Sale of Stock [Axis] Beginning balance Ending balance Fair Value, Measurement with Unobservable Inputs Reconciliation, Recurring Basis, Asset Value Dividends [Axis] Dividends [Axis] Drawn revolvers Drawn Revolvers [Member] Drawn Revolvers New Mountain Net Lease Corporation NMNLC New Mountain Net Lease Corporation [Member] New Mountain Net Lease Corporation Cash flows from financing activities Net Cash Provided by (Used in) Financing Activities [Abstract] Asset Class [Axis] Asset Class [Axis] Earnings per share Earnings Per Share, Policy [Policy Text Block] CHA Holdings, Inc. CHA Holdings, Inc. [Member] CHA Holdings, Inc. Variable Rate [Axis] Variable Rate [Axis] Ratio to average net assets: Investment Company, Ratio To Average Net Assets [Abstract] Investment Company, Ratio To Average Net Assets Other liabilities Increase (Decrease) in Other Operating Liabilities Weighted average Weighted Average Weighted Average [Member] Investments at fair value that are non-qualifying assets as a percentage of total assets Investment Owned, At Fair Value, Non-Qualifying Assets, Percent Of Total Assets Investment Owned, At Fair Value, Non-Qualifying Assets, Percent Of Total Assets NM CLFX LP NM CLFX LP [Member] NM CLFX LP Schedule of Basic and Diluted Earnings Per Share Schedule of Earnings Per Share, Basic and Diluted [Table Text Block] Diligent Corporation Diligent Corporation [Member] Diligent Corporation Schedule of Information Related to Convertible Notes Convertible Debt [Table Text Block]