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Financial Highlights - Senior Securities (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands | 12 Months Ended |  |  |  |  |  |
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Dec. 31, 2024 EUR (€) $ / shares | Dec. 31, 2023 EUR (€) $ / shares | Dec. 31, 2022 EUR (€) $ / shares | Dec. 31, 2021 EUR (€) $ / shares | Dec. 31, 2020 USD ($) $ / shares | Dec. 31, 2019 USD ($) $ / shares | Dec. 31, 2018 USD ($) $ / shares | Dec. 31, 2017 USD ($) $ / shares | Dec. 31, 2016 USD ($) $ / shares | Dec. 31, 2015 USD ($) $ / shares | Dec. 31, 2024 GBP (£) | Dec. 31, 2024 USD ($) | Dec. 31, 2023 GBP (£) | Dec. 31, 2023 USD ($) | Nov. 13, 2023 | Dec. 31, 2022 GBP (£) | Dec. 31, 2022 USD ($) | Dec. 31, 2021 GBP (£) | Dec. 31, 2021 USD ($) | Sep. 25, 2018
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
SBA-guaranteed debentures outstanding |  |  |  |  | $ 300,000 | $ 225,000 | $ 165,000 | $ 150,000 | $ 121,700 | $ 117,700 |  | $ 300,000 |  | $ 300,000 |  |  | $ 300,000 |  | $ 300,000 |
SBA-guaranteed debentures, coverage per unit (in dollars per share) | $ / shares | $ 1,727 | $ 1,730 | $ 1,658 | $ 1,686 | $ 1,673 | $ 1,655 | $ 1,718 | $ 2,169 | $ 2,320 | $ 2,128 |  |  |  |  |  |  |  |  |  |
Holdings Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance |  |  |  |  |  |  |  |  |  |  |  | $ 294,363 |  | 515,063 |  |  | 618,963 |  |  |
8.25% Notes due 2028 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 8.25%
8.25% Notes due 2028 | Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate | 8.25% |  |  |  |  |  |  |  |  |  | 8.25% | 8.25% |  |  | 8.25% |  |  |  |  | 8.25%
NMFC Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance |  |  |  |  |  |  |  |  |  |  |  | $ 27,944 |  | 36,813 |  |  |  |  |  |
NMFC Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance | $ 16,512 | $ 700 | $ 700 | $ 0 |  |  |  |  |  |  | £ 8,700 |  | £ 22,900 |  |  | £ 22,900 | 40,359 | £ 16,400 |  |
DB Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 186,400 |  |  |  |  |  |
DB Credit Facility | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 186,400 |  |  |
NMNLC Credit Facility II |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance |  |  |  |  |  |  |  |  |  |  |  | 0 |  | 2,853 |  |  |  |  |  |
NMNLC Credit Facility II | Revolving Credit Facility |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Credit facility outstanding balance |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  | $ 2,853 |  |  | $ 3,785 |  |  |
5.75% Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.75%
5.75% Unsecured Notes | Unsecured Notes |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Debt Instrument [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Stated interest rate |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5.75%

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