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Financial Highlights (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands | 3 Months Ended |  |  |
--- | --- | --- | --- | ---
Mar. 31, 2024 USD ($) $ / shares shares | Mar. 31, 2023 USD ($) $ / shares shares | Mar. 31, 2024 GBP (£) shares | Mar. 31, 2024 EUR (€) shares | Dec. 31, 2023 USD ($) shares | Dec. 31, 2023 GBP (£) shares | Dec. 31, 2023 EUR (€) shares | Mar. 31, 2023 GBP (£) shares | Mar. 31, 2023 EUR (€) shares | Mar. 31, 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 | 0.36 | 0.38 |  |  |  |  |  |  |  |
Net realized and unrealized gains (in dollars per share) | $ / shares | (0.10) | 0.06 |  |  |  |  |  |  |  |
Total net increase (in dollars per share) | $ / shares | 0.26 | 0.44 |  |  |  |  |  |  |  |
Distributions declared to stockholders from net investment income (in dollars per share) | $ / shares | (0.36) | (0.32) |  |  |  |  |  |  |  |
Net asset value, ending balance (in dollars per share) | $ / shares | 12.77 | $ 13.14 |  |  |  |  |  |  |  |
Per share market value (in dollars per share) | $ / shares | $ 12.67 |  |  |  |  |  |  |  |  | $ 12.17
Total return based on market value | 2.45% | 0.94% |  |  |  |  |  |  |  |
Total return based on net asset value | 1.99% | 3.39% |  |  |  |  |  |  |  |
Shares outstanding at end of period (in shares) | shares | 106,289,293 |  | 106,289,293 | 106,289,293 | 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 | 103,660,370 | 100,937,026 |  |  |  |  |  |  |  |
Average net assets for the period | $ 1,334,024 | $ 1,314,609 |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Net investment income | 11.25% | 11.76% |  |  |  |  |  |  |  |
Total expenses, before waivers/reimbursements | 16.25% | 16.85% |  |  |  |  |  |  |  |
Total expenses, net of waivers/reimbursements | 15.97% | 16.52% |  |  |  |  |  |  |  |
Asset coverage ratio | 192.25% |  | 192.25% | 192.25% |  |  |  | 177.55% | 177.55% |
Portfolio turnover | 4.83% | 2.10% |  |  |  |  |  |  |  |
Unsecured Notes |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Average debt outstanding | $ 632,610 | $ 471,500 |  |  |  |  |  |  |  |
SBA-guaranteed debentures |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 300,000 | 300,000 |  |  |  |  |  |  |  |
Convertible Notes |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 260,000 | 328,816 |  |  |  |  |  |  |  |
Holdings Credit Facility |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 291,563 |  |  |  | $ 515,063 |  |  |  |  |
Holdings Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 291,563 |  |  |  | 515,063 |  |  |  |  |
Holdings Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 358,489 | 634,146 |  |  |  |  |  |  |  |
DB Credit Facility |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 182,000 |  |  |  | 186,400 |  |  |  |  |
DB Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 182,000 |  |  |  | 186,400 |  |  |  |  |
DB Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 185,095 | 186,400 |  |  |  |  |  |  |  |
NMFC Credit Facility |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 47,618 |  |  |  | 36,813 |  |  |  |  |
NMFC Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 47,618 |  | £ 22,850 | € 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 | 35,729 | 101,025 |  |  |  |  |  |  |  |
NMNLC Credit Facility II |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 2,938 |  |  |  | 2,853 |  |  |  |  |
NMNLC Credit Facility II | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 2,938 |  |  |  | $ 2,853 |  |  |  |  |
NMNLC Credit Facility II | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |
Average debt outstanding | $ 2,371 | $ 2,973 |  |  |  |  |  |  |  |

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