Financial Highlights (Tables)
 9 Months Ended
Sep. 30, 2022
Investment Company [Abstract]  
Schedule of Financial Highlights
The following information sets forth the Company's financial highlights for the nine months ended September 30, 2022 and September 30, 2021:
 Nine Months Ended
 September 30, 2022September 30, 2021
Per share data(1):  
Net asset value, January 1, 2022 and January 1, 2021, respectively$13.49 $12.62 
Net investment income0.94 0.91 
Net realized and unrealized (losses) gains (2)(0.33)0.63 
Total net increase0.61 1.54 
Distributions declared to stockholders from net investment income(0.90)(0.90)
Net asset value, September 30, 2022 and September 30, 2021, respectively$13.20 $13.26 
Per share market value, September 30, 2022 and September 30, 2021, respectively$11.53 $13.31 
Total return based on market value(3)(9.64)%25.41 %
Total return based on net asset value(4)4.54 %12.42 %
Shares outstanding at end of period100,937,026 96,906,988 
Average weighted shares outstanding for the period99,955,432 96,854,474 
Average net assets for the period$1,348,479 $1,253,263 
Ratio to average net assets:  
Net investment income9.27 %9.48 %
Total expenses, before waivers/reimbursements12.85 %13.35 %
Total expenses, net of waivers/reimbursements12.50 %12.15 %
Average debt outstanding—Holdings Credit Facility$580,699 $467,570 
Average debt outstanding—Unsecured Notes525,456 518,333 
Average debt outstanding—SBA-guaranteed debentures300,000 300,000 
Average debt outstanding—DB Credit Facility217,817 209,417 
Average debt outstanding—2018 Convertible Notes201,250 201,250 
Average debt outstanding—NMFC Credit Facility(5)156,303 122,537 
Average debt outstanding—NMNLC Credit Facility II(6)8,939 1,533 
Asset coverage ratio(7)179.28 %183.99 %
Portfolio turnover14.38 %25.47 %
(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)Includes the effective of common stock issuances per share, which for the nine months ended September 30, 2022 and September 30, 2021 were $0.01 and $0.00, respectively.
(3)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.
(4)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.
(5)Under the NMFC Credit Facility, the Company may borrow in U.S. dollars or certain other permitted currencies. As of September 30, 2022, the Company had borrowings denominated in GBP of £22,850 and borrowings denominated in EUR of €700 that has been converted to U.S. dollars.
(6)For the nine months ended September 30, 2021, average debt outstanding represents the period from February 26, 2021 (commencement of the NMNLC Credit Facility II) to September 30, 2021.
(7)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.