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Fair Value - Principal Amounts and Fair Values of Borrowings (Details) € in Thousands, £ in Thousands, $ in Thousands | Sep. 30, 2024 USD ($) | Sep. 30, 2024 GBP (£) | Sep. 30, 2024 EUR (€) | Dec. 31, 2023 USD ($) | Dec. 31, 2023 GBP (£) | Dec. 31, 2023 EUR (€)
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Principal Amount |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | $ 2,015,502 |  |  | $ 1,807,629 |  |
Principal Amount | Unsecured Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 990,000 |  |  | 506,500 |  |
Principal Amount | SBA-guaranteed debentures |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 300,000 |  |  | 300,000 |  |
Principal Amount | Convertible Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 260,000 |  |  | 260,000 |  |
Principal Amount | Revolving Credit Facility | Holdings Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 407,563 |  |  | 515,063 |  |
Principal Amount | Revolving Credit Facility | NMFC Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 55,031 | £ 26,650 | € 17,400 | 36,813 | £ 22,850 | € 700
Principal Amount | Revolving Credit Facility | NMNLC Credit Facility II | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 2,908 |  |  | 2,853 |  |
Principal Amount | Revolving Credit Facility | DB Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 0 |  |  | 186,400 |  |
Fair Value |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 1,985,710 |  |  | 1,750,087 |  |
Fair Value | Unsecured Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 988,569 |  |  | 490,200 |  |
Fair Value | SBA-guaranteed debentures |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 270,806 |  |  | 259,811 |  |
Fair Value | Convertible Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 263,250 |  |  | 264,706 |  |
Fair Value | Revolving Credit Facility | Holdings Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 406,072 |  |  | 511,511 |  |
Fair Value | Revolving Credit Facility | NMFC Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 54,105 | £ 26,659 | € 16,557 | 36,507 | £ 22,660 | € 694
Fair Value | Revolving Credit Facility | NMNLC Credit Facility II | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 2,908 |  |  | 2,846 |  |
Fair Value | Revolving Credit Facility | DB Credit Facility | Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | $ 0 |  |  | $ 184,506 |  |

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