Financial Highlights (Tables)
 6 Months Ended
Jun. 30, 2023
Investment Company [Abstract]  
Schedule of Financial Highlights
The following information sets forth the Company's financial highlights for the six months ended June 30, 2023 and June 30, 2022:
 Six Months Ended
 June 30, 2023June 30, 2022
Per share data(1):  
Net asset value, January 1, 2023 and January 1, 2022, respectively$13.02 $13.49 
Net investment income0.77 0.61 
Net realized and unrealized gains0.02 (0.08)
Total net increase0.79 0.53 
Distributions declared to stockholders from net investment income(0.67)(0.60)
Net asset value, June 30, 2023 and June 30, 2022, respectively$13.14 $13.42 
Per share market value, June 30, 2023 and June 30, 2022, respectively$12.44 $11.91 
Total return based on market value(2)6.04 %(9.00)%
Total return based on net asset value(3)6.09 %3.95 %
Shares outstanding at end of period100,937,026 100,716,928 
Average weighted shares outstanding for the period100,937,026 99,510,862 
Average net assets for the period$1,320,680 $1,346,043 
Ratio to average net assets(4):  
Net investment income11.88 %9.13 %
Total expenses, before waivers/reimbursements16.99 %12.42 %
Total expenses, net of waivers/reimbursements16.66 %12.05 %
Average debt outstanding—Holdings Credit Facility$623,325 $567,033 
Average debt outstanding—Unsecured Notes455,091 518,130 
Average debt outstanding—SBA-guaranteed debentures300,000 300,000 
Average debt outstanding—DB Credit Facility186,400 227,156 
Average debt outstanding—Convertible Notes352,949 201,250 
Average debt outstanding—NMFC Credit Facility(5)85,148 148,401 
Average debt outstanding—NMNLC Credit Facility II2,678 12,459 
Asset coverage ratio(6)181.68 %178.75 %
Portfolio turnover3.84 %9.10 %
(1)Per share data is based on weighted average shares outstanding for the respective period (except for distributions declared to stockholders, which is based on actual rate per share).
(2)Total return is calculated assuming a purchase of common stock at the opening of the first day of the year and a sale on the closing of the last business day of the period. Dividends and distributions, if any, are assumed for purposes of this calculation, to be reinvested at prices obtained under the Company’s dividend reinvestment plan. Total return does not reflect sales load.
(3)Total return is calculated assuming a purchase at net asset value on the opening of the first day of the year and a sale at net asset value on the last day of the period. Dividends and distributions, if any, are assumed for purposes of this calculation, to be reinvested at the net asset value on the last day of the respective quarter. Total return does not reflect sales load.
(4)Interim periods are annualized.
(5)Under the NMFC Credit Facility, the Company may borrow in U.S. dollars or certain other permitted currencies. As of June 30, 2023 and June 30, 2022, the Company had borrowings denominated in GBP of £22,850 and £18,800, respectively, and borrowings denominated in EUR of €700 and €0, respectively, that have been converted to U.S. dollars.
(6)On November 5, 2014, the Company received exemptive relief from the SEC allowing the Company to modify the asset coverage requirement to exclude the SBA-guaranteed debentures from this calculation.