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Financial Highlights (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands | 3 Months Ended | 9 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Sep. 30, 2024 USD ($) $ / shares shares | Sep. 30, 2023 USD ($) $ / shares | Sep. 30, 2024 USD ($) $ / shares shares | Sep. 30, 2023 USD ($) $ / shares shares | Sep. 30, 2024 GBP (£) shares | Sep. 30, 2024 EUR (€) shares | Dec. 31, 2023 USD ($) shares | Dec. 31, 2023 GBP (£) shares | Dec. 31, 2023 EUR (€) shares | Sep. 30, 2023 GBP (£) shares | Sep. 30, 2023 EUR (€) shares | Sep. 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 |  |  | 1.05 | 1.17 |  |  |  |  |  |  |  |
Net realized and unrealized losses (in dollars per share) | $ / shares |  |  | (0.26) | (0.10) |  |  |  |  |  |  |  |
Total net increase (in dollars per share) | $ / shares |  |  | 0.79 | 1.07 |  |  |  |  |  |  |  |
Distributions declared to stockholders from net investment income (in dollars per share) | $ / shares |  |  | (1.04) | (1.03) |  |  |  |  |  |  |  |
Net asset value, ending balance (in dollars per share) | $ / shares | $ 12.62 | $ 13.06 | 12.62 | $ 13.06 |  |  |  |  |  |  |  |
Per share market value (in dollars per share) | $ / shares | $ 11.98 |  | $ 11.98 |  |  |  |  |  |  |  |  | $ 12.95
Total return based on market value |  |  | 2.40% | 13.49% |  |  |  |  |  |  |  |
Total return based on net asset value |  |  | 6.32% | 8.39% |  |  |  |  |  |  |  |
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 | 101,439,649 | 101,439,649 |
Average weighted shares outstanding for the period (in shares) | shares |  |  | 106,140,789 | 100,943,049 |  |  |  |  |  |  |  |
Average net assets for the period |  |  | $ 1,357,569 | $ 1,322,558 |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  | 10.93% | 11.95% |  |  |  |  |  |  |  |
Total expenses, before waivers/reimbursements |  |  | 16.90% | 16.77% |  |  |  |  |  |  |  |
Total expenses, net of waivers/reimbursements |  |  | 16.63% | 16.45% |  |  |  |  |  |  |  |
Asset coverage ratio | 179.18% |  | 179.18% |  | 179.18% | 179.18% |  |  |  | 182.68% | 182.68% |
Portfolio turnover |  |  | 17.99% | 4.36% |  |  |  |  |  |  |  |
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 | $ 706,304 | $ 391,500 | $ 676,414 | $ 433,661 |  |  |  |  |  |  |  |
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 | 317,138 | 260,000 | 340,881 |  |  |  |  |  |  |  |
Holdings Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 407,563 |  | 407,563 |  |  |  | $ 515,063 |  |  |  |  |
Holdings Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 407,563 |  | 407,563 |  |  |  | 515,063 |  |  |  |  |
Holdings Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 405,661 | 563,470 | 364,418 | 603,154 |  |  |  |  |  |  |  |
DB Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 0 |  | 0 |  |  |  | 186,400 |  |  |  |  |
DB Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 0 |  | 0 |  |  |  | 186,400 |  |  |  |  |
DB Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 180,022 | 186,487 | 182,364 | 186,429 |  |  |  |  |  |  |  |
NMFC Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 55,031 |  | 55,031 |  |  |  | 36,813 |  |  |  |  |
NMFC Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 55,031 |  | 55,031 |  | £ 26,650 | € 17,400 | 36,813 | £ 22,850 | € 700 | £ 22,850 | € 700 |
NMFC Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 140,385 | 141,764 | 79,969 | 104,227 |  |  |  |  |  |  |  |
NMNLC Credit Facility II |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 2,908 |  | 2,908 |  |  |  | 2,853 |  |  |  |  |
NMNLC Credit Facility II | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 2,908 |  | 2,908 |  |  |  | $ 2,853 |  |  |  |  |
NMNLC Credit Facility II | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | $ 2,151 | $ 2,091 | $ 2,341 | $ 2,480 |  |  |  |  |  |  |  |

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