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Financial Highlights - Summary (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands | 12 Months Ended |  |  |  |  |
--- | --- | --- | --- | --- | --- | ---
Dec. 31, 2023 USD ($) $ / shares shares | Dec. 31, 2022 USD ($) $ / shares shares | Dec. 31, 2021 USD ($) $ / shares shares | Dec. 31, 2020 USD ($) $ / shares shares | Dec. 31, 2019 USD ($) $ / shares shares | Dec. 31, 2023 GBP (£) shares | Dec. 31, 2023 EUR (€) shares | Nov. 13, 2023 | Dec. 31, 2022 GBP (£) shares | Dec. 31, 2022 EUR (€) shares | Dec. 31, 2021 GBP (£) shares | Dec. 31, 2021 EUR (€) shares | Sep. 25, 2018
Investment Company, Financial Highlights [Roll Forward] |  |  |  |  |  |  |  |  |  |  |  |  |
Net asset value, beginning balance (in dollars per share) | $ / shares | $ 13.02 | $ 13.49 | $ 12.62 | $ 13.26 | $ 13.22 |  |  |  |  |  |  |  |
Net investment income (in dollars per share) | $ / shares | 1.57 | 1.18 | 1.21 | 1.20 | 1.37 |  |  |  |  |  |  |  |
Net realized and unrealized (losses) gains (in dollars per share) | $ / shares | (0.23) | (0.43) | 0.86 | (0.60) | 0.03 |  |  |  |  |  |  |  |
Total net increase (in dollars per share) | $ / shares | 1.34 | 0.75 | 2.07 | 0.60 | 1.40 |  |  |  |  |  |  |  |
Distributions declared to stockholders from net investment income (in dollars per share) | $ / shares | (1.49) | (1.22) | (1.20) | (1.24) | (1.36) |  |  |  |  |  |  |  |
Net asset value, ending balance (in dollars per share) | $ / shares | 12.87 | 13.02 | 13.49 | 12.62 | 13.26 |  |  |  |  |  |  |  |
Per share market value (in dollars per share) | $ / shares | $ 12.72 | $ 12.37 | $ 13.70 | $ 11.36 | $ 13.74 |  |  |  |  |  |  |  |
Total return based on market value | 15.48% | (0.56%) | 31.91% | (5.24%) | 20.45% |  |  |  |  |  |  |  |
Total return based on net asset value | 10.64% | 5.71% | 16.97% | 5.52% | 10.90% |  |  |  |  |  |  |  |
Shares outstanding at end of period (in shares) | shares | 102,558,859 | 100,937,026 | 97,907,441 | 96,827,342 | 96,827,342 | 102,558,859 | 102,558,859 |  | 100,937,026 | 100,937,026 | 97,907,441 | 97,907,441 |
Average weighted shares outstanding for the period (in shares) | shares | 101,118,302 | 100,202,847 | 96,952,959 | 96,827,342 | 85,209,378 |  |  |  |  |  |  |  |
Average net assets for the period | $ 1,323,778 | $ 1,344,266 | $ 1,261,338 | $ 1,168,043 | $ 1,154,615 |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Net investment income | 12.00% | 8.82% | 9.32% | 10.05% | 10.15% |  |  |  |  |  |  |  |
Total expenses, before waivers/reimbursements | 16.54% | 13.35% | 13.11% | 14.56% | 14.87% |  |  |  |  |  |  |  |
Total expenses, net of waivers/reimbursements | 16.23% | 13.01% | 12.05% | 13.39% | 13.80% |  |  |  |  |  |  |  |
Asset coverage ratio | 187.54% | 177.42% | 181.21% | 180.68% | 173.98% | 187.54% | 187.54% |  | 177.42% | 177.42% | 181.21% | 181.21% |
Portfolio turnover | 8.87% | 18.01% | 35.33% | 15.43% | 11.58% |  |  |  |  |  |  |  |
Effect of common stock issuance (in dollars per share) | $ / shares | $ 0.00 | $ 0.01 | $ (0.01) | $ 0.00 | $ 0.08 |  |  |  |  |  |  |  |
Convertible Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | $ 320,494 | $ 228,806 | $ 201,250 | $ 201,250 | $ 234,332 |  |  |  |  |  |  |  |
SBA-guaranteed debentures |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 300,000 | 300,000 | 300,000 | 285,852 | 179,408 |  |  |  |  |  |  |  |
Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 438,473 | 526,829 | 516,611 | 453,250 | 414,949 |  |  |  |  |  |  |  |
Holdings Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 515,063 | 618,963 | 545,263 |  |  |  |  |  |  |  |  |  |
Holdings Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 577,759 | 581,367 | 478,016 | 526,645 | 598,129 |  |  |  |  |  |  |  |
NMFC Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 36,813 | 40,359 |  |  |  |  |  |  |  |  |  |  |
NMFC Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance |  |  | 127,192 |  |  | £ 22,900 | € 700 |  | £ 22,900 | € 700 | £ 16,400 | € 0 |
NMFC Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 103,244 | 133,053 | 132,685 | 155,497 | 105,533 |  |  |  |  |  |  |  |
DB Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 186,400 | 186,400 |  |  |  |  |  |  |  |  |  |  |
DB Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance |  |  | 244,000 |  |  |  |  |  |  |  |  |  |
DB Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 186,422 | 209,898 | 209,307 | 233,649 | 113,967 |  |  |  |  |  |  |  |
NMNLC Credit Facility | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | 0 | 0 | 0 | 0 | 1,471 |  |  |  |  |  |  |  |
NMNLC Credit Facility II |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 2,853 | 3,785 |  |  |  |  |  |  |  |  |  |  |
NMNLC Credit Facility II | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | 2,853 | 3,785 | 15,200 |  |  |  |  |  |  |  |  |  |
NMNLC Credit Facility II | Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Average debt outstanding | $ 2,506 | $ 7,195 | $ 3,501 | $ 0 | $ 0 |  |  |  |  |  |  |  |
8.25% Notes due 2028 | Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |  |
Ratio to average net assets: |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  | 8.25% |  |  |  |  | 8.25%

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