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Financial Highlights (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands | 3 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2024 USD ($) $ / shares shares | Jun. 30, 2023 USD ($) $ / shares | Jun. 30, 2024 USD ($) $ / shares shares | Jun. 30, 2023 USD ($) $ / shares shares | Jun. 30, 2024 EUR (€) shares | Jun. 30, 2024 GBP (£) shares | Dec. 31, 2023 USD ($) shares | Dec. 31, 2023 EUR (€) shares | Dec. 31, 2023 GBP (£) shares | Jun. 30, 2023 EUR (€) shares | Jun. 30, 2023 GBP (£) shares | Jun. 30, 2023 $ / shares
Investment Company, Financial Highlights [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |
Net asset value, beginning balance (in dollars per share) | $ / shares |  |  | $ 12.87 | $ 13.02 |  |  |  |  |  |  |  |
Net investment income (in dollars per share) | $ / shares |  |  | 0.72 | 0.77 |  |  |  |  |  |  |  |
Net realized and unrealized gains (losses) (in dollars per share) | $ / shares |  |  | (0.15) | 0.02 |  |  |  |  |  |  |  |
Total net increase (in dollars per share) | $ / shares |  |  | 0.57 | 0.79 |  |  |  |  |  |  |  |
Distributions declared to stockholders from net investment income (in dollars per share) | $ / shares |  |  | (0.70) | (0.67) |  |  |  |  |  |  |  |
Net asset value, ending balance (in dollars per share) | $ / shares | $ 12.74 | $ 13.14 | 12.74 | $ 13.14 |  |  |  |  |  |  |  |
Per share market value (in dollars per share) | $ / shares | $ 12.24 |  | $ 12.24 |  |  |  |  |  |  |  |  | $ 12.44
Total return based on market value |  |  | 1.71% | 6.04% |  |  |  |  |  |  |  |
Total return based on net asset value |  |  | 4.50% | 6.09% |  |  |  |  |  |  |  |
Shares outstanding at end of period (in shares) | shares | 107,851,415 |  | 107,851,415 |  | 107,851,415 | 107,851,415 | 102,558,859 | 102,558,859 | 102,558,859 | 100,937,026 | 100,937,026 |
Average weighted shares outstanding for the period - basic (in shares) | shares |  |  | 105,276,077 | 100,937,026 |  |  |  |  |  |  |  |
Average net assets for the period |  |  | $ 1,349,273 | $ 1,320,680 |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  | 11.26% | 11.88% |  |  |  |  |  |  |  |
Total expenses, before waivers/reimbursements |  |  | 16.53% | 16.99% |  |  |  |  |  |  |  |
Total expenses, net of waivers/reimbursements |  |  | 16.26% | 16.66% |  |  |  |  |  |  |  |
Asset coverage ratio | 182.80% |  | 182.80% |  | 182.80% | 182.80% |  |  |  | 181.68% | 181.68% |
Portfolio turnover |  |  | 14.94% | 3.84% |  |  |  |  |  |  |  |
Accretive effect of common stock issuances per share (in dollars per share) | $ / shares |  |  | $ (0.02) | $ 0.00 |  |  |  |  |  |  |  |
Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | $ 690,000 | $ 439,852 | $ 661,305 | $ 455,091 |  |  |  |  |  |  |  |
SBA-guaranteed debentures |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 300,000 | 300,000 | 300,000 | 300,000 |  |  |  |  |  |  |  |
Convertible Notes |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 260,000 | 376,816 | 260,000 | 352,949 |  |  |  |  |  |  |  |
Holdings Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 386,563 |  | 386,563 |  |  |  | $ 515,063 |  |  |  |  |
Holdings Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 386,563 |  | 386,563 |  |  |  | 515,063 |  |  |  |  |
Holdings Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 328,651 | 612,623 | 343,570 | 623,325 |  |  |  |  |  |  |  |
DB Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 182,000 |  | 182,000 |  |  |  | 186,400 |  |  |  |  |
DB Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 182,000 |  | 182,000 |  |  |  | 186,400 |  |  |  |  |
DB Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 182,000 | 186,400 | 183,547 | 186,400 |  |  |  |  |  |  |  |
NMFC Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 141,840 |  | 141,840 |  |  |  | 36,813 |  |  |  |  |
NMFC Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 141,840 |  | 141,840 |  | € 17,400 | £ 26,650 | 36,813 | € 700 | £ 22,850 | € 700 | £ 22,850 |
NMFC Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 63,131 | 69,445 | 49,430 | 85,148 |  |  |  |  |  |  |  |
NMNLC Credit Facility II |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 2,948 |  | 2,948 |  |  |  | 2,853 |  |  |  |  |
NMNLC Credit Facility II | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 2,948 |  | 2,948 |  |  |  | $ 2,853 |  |  |  |  |
NMNLC Credit Facility II | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | $ 2,505 | $ 2,386 | $ 2,438 | $ 2,678 |  |  |  |  |  |  |  |

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