Consolidated Statements of Cash Flows - USD ($)
$ in Thousands
 3 Months Ended 9 Months Ended 136 Months Ended
Sep. 30, 2022
Mar. 31, 2022
Sep. 30, 2021
Mar. 31, 2021
Sep. 30, 2022
Sep. 30, 2021
Sep. 30, 2022
Cash flows from operating activities              
Net increase in net assets resulting from operations $ 7,514 $ 37,047 $ 22,905 $ 51,855 $ 59,701 $ 153,938  
Adjustments to reconcile net increase in net assets resulting from operations to net cash (used in) provided by operating activities:              
Net realized gains on investments         (35,468) (12,692)  
Net realized gains on translation of assets and liabilities in foreign currencies 166   0   (219) 0  
Net change in unrealized depreciation (appreciation) of investments         67,634 (52,544)  
Net change in unrealized depreciation on translation of assets and liabilities in foreign currencies 10   13   625 13  
Net change in unrealized depreciation of securities purchased under collateralized agreements to resell 0   0   2,021 0  
Amortization of purchase discount         (4,437) (6,681)  
Amortization of deferred financing costs         5,082 5,660  
Amortization of premium on Convertible Notes         (77) (77)  
Non-cash investment income         (35,585) (33,226)  
(Increase) decrease in operating assets:              
Purchase of investments and delayed draw facilities         (526,743) (769,167)  
Proceeds from sales and paydowns of investments         466,516 816,218  
Cash received for purchase of undrawn portion of revolving credit or delayed draw facilities         321 597  
Cash paid for purchase of drawn portion of revolving credit facilities         (185) (832)  
Cash paid on drawn revolvers         (30,438) (25,310)  
Cash repayments on drawn revolvers         33,589 24,980  
Deferred tax asset         0 101  
Interest and dividend receivable         (3,050) (4,362)  
Receivable from unsettled securities sold         0 29  
Receivable from affiliates         0 117  
Other assets         (275) (3,907)  
Increase (decrease) in operating liabilities:              
Management fee payable         438 (431)  
Incentive fee payable         699 307  
Payable for unsettled securities purchased         (7,910) (2,184)  
Payable to affiliates         (281) (551)  
Interest payable         (5,174) (6,059)  
Deferred tax liability         127 13  
Other liabilities         4,240 490  
Contributions related to non-controlling interest in New Mountain Net Lease Corporation ("NMNLC")         0 250  
Net cash flows (used in) provided by operating activities         (8,849) 84,690  
Cash flows from financing activities              
Net proceeds from shares sold         40,006 0 $ 945,617
Offering costs paid         (132) 0  
Distributions paid         (88,987) (86,119)  
Proceeds from Unsecured Notes         75,000 200,000  
Repayment of Unsecured Notes         (55,000) (141,750)  
Contributions related to non-controlling interest in NMNLC         124 0  
Distributions related to non-controlling interest in NMNLC         (8,197) 0  
Deferred financing costs paid         (710) (10,144)  
Net cash flows provided by (used in) by financing activities         45 (80,405)  
Net (decrease) increase in cash and cash equivalents         (8,804) 4,285  
Effect of foreign exchange rate changes on cash and cash equivalents         (354) 106  
Cash and cash equivalents at the beginning of the period   $ 58,077   $ 78,966 58,077 78,966  
Cash and cash equivalents at the end of the period $ 48,919   $ 83,357   48,919 83,357 $ 48,919
Supplemental disclosure of cash flow information              
Cash interest paid         63,006 53,643  
Income taxes paid         74 15  
Non-cash financing activities:              
Value of shares issued/ reissued from repurchase program in connection with the distribution reinvestment plan         1,098 1,049  
Accrual for offering costs         30 27  
Accrual for deferred financing costs         15 14  
Holdings Credit Facility              
Cash flows from financing activities              
Proceeds from Credit Facility         164,400 129,000  
Repayments of Credit Facility         (79,000) (85,900)  
NMFC Credit Facility              
Cash flows from financing activities              
Proceeds from Credit Facility         162,707 311,363  
Repayments of Credit Facility         (158,000) (326,500)  
DB Credit Facility              
Cash flows from financing activities              
Proceeds from Credit Facility         108,600 77,500  
Repayments of Credit Facility         (148,500) (153,700)  
NMNLC Credit Facility II              
Cash flows from financing activities              
Proceeds from Credit Facility         2,934 9,025  
Repayments of Credit Facility         $ (15,200) $ (3,180)