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Financial Highlights (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands | 3 Months Ended | 6 Months Ended |  |  |
--- | --- | --- | --- | --- | ---
Jun. 30, 2025 USD ($) $ / shares shares | Jun. 30, 2024 USD ($) $ / shares | Jun. 30, 2025 USD ($) $ / shares shares | Jun. 30, 2024 USD ($) $ / shares shares | Jun. 30, 2025 EUR (€) shares | Jun. 30, 2025 GBP (£) shares | Dec. 31, 2024 USD ($) shares | Dec. 31, 2024 EUR (€) shares | Dec. 31, 2024 GBP (£) shares | Jun. 30, 2024 EUR (€) shares | Jun. 30, 2024 GBP (£) shares
Investment Company, Financial Highlights [Roll Forward] |  |  |  |  |  |  |  |  |  |  |
Net asset value, beginning balance (in dollars per share) | $ / shares |  |  | $ 12.55 | $ 12.87 |  |  |  |  |  |  |
Net investment income (in dollars per share) | $ / shares |  |  | 0.64 | 0.72 |  |  |  |  |  |  |
Net realized and unrealized losses (in dollars per share) | $ / shares |  |  | (0.36) | (0.15) |  |  |  |  |  |  |
Total net increase (in dollars per share) | $ / shares |  |  | 0.28 | 0.57 |  |  |  |  |  |  |
Repurchase of common stock (in dollars per share) | $ / shares |  |  | 0.02 | 0 |  |  |  |  |  |  |
Distributions declared to stockholders from net investment income (in dollars per share) | $ / shares |  |  | (0.64) | (0.70) |  |  |  |  |  |  |
Net asset value, ending balance (in dollars per share) | $ / shares | $ 12.21 | $ 12.74 | 12.21 | 12.74 |  |  |  |  |  |  |
Per share market value (in dollars per share) | $ / shares | $ 10.55 | $ 12.24 | $ 10.55 | $ 12.24 |  |  |  |  |  |  |
Total return based on market value |  |  | (0.72%) | 1.71% |  |  |  |  |  |  |
Total return based on net asset value |  |  | 2.44% | 4.50% |  |  |  |  |  |  |
Shares outstanding at end of period (in shares) | shares | 106,926,713 |  | 106,926,713 |  | 106,926,713 | 106,926,713 | 107,851,415 | 107,851,415 | 107,851,415 | 107,851,415 | 107,851,415
Average weighted shares outstanding for the period (in shares) | shares |  |  | 107,800,508 | 105,276,077 |  |  |  |  |  |  |
Average net assets for the period |  |  | $ 1,347,006 | $ 1,349,273 |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Net investment income |  |  | 10.33% | 11.26% |  |  |  |  |  |  |
Total expenses, before waivers/reimbursements |  |  | 15.62% | 16.53% |  |  |  |  |  |  |
Total expenses, net of waivers/reimbursements |  |  | 14.96% | 16.26% |  |  |  |  |  |  |
Asset coverage ratio | 185.56% |  | 185.56% |  | 185.56% | 185.56% |  |  |  | 182.80% | 182.80%
Portfolio turnover |  |  | 7.94% | 14.94% |  |  |  |  |  |  |
Unsecured Notes |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | $ 990,000 | $ 690,000 | $ 990,000 | $ 661,305 |  |  |  |  |  |  |
SBA-guaranteed debentures |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 262,500 | 300,000 | 274,724 | 300,000 |  |  |  |  |  |  |
2022 Convertible Notes |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 258,784 | 260,000 | 259,147 | 260,000 |  |  |  |  |  |  |
Holdings Credit Facility |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 246,063 |  | 246,063 |  |  |  | $ 294,363 |  |  |  |
Holdings Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 246,063 |  | 246,063 |  |  |  | 294,363 |  |  |  |
Holdings Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 251,167 | 328,651 | 280,737 | 343,570 |  |  |  |  |  |  |
NMFC Credit Facility |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 31,372 |  | 31,372 |  |  |  | 27,944 |  |  |  |
NMFC Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 31,372 |  | 31,372 |  | € 16,512 | £ 8,666 | $ 27,944 | € 16,512 | £ 8,666 | € 17,400 | £ 22,850
NMFC Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | $ 30,315 | 63,131 | 29,314 | 49,430 |  |  |  |  |  |  |
DB Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding |  | 182,000 | 0 | 183,547 |  |  |  |  |  |  |
NMNLC Credit Facility II | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding |  | $ 2,505 | $ 0 | $ 2,438 |  |  |  |  |  |  |

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