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Fair Value - Principal Amounts and Fair Values of Borrowings (Details) € in Thousands, £ in Thousands, $ in Thousands | Mar. 31, 2025 USD ($) | Mar. 31, 2025 EUR (€) | Mar. 31, 2025 GBP (£) | Dec. 31, 2024 USD ($) | Dec. 31, 2024 EUR (€) | Dec. 31, 2024 GBP (£)
--- | --- | --- | --- | --- | --- | ---
Principal Amount |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | $ 1,810,906 |  |  | $ 1,872,307 |  |
Principal Amount | Unsecured Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 990,000 |  |  | 990,000 |  |
Principal Amount | Credit Facility | Holdings Credit Facility | Revolving Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 270,563 |  |  | 294,363 |  |
Principal Amount | Credit Facility | NMFC Credit Facility | Revolving Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 29,059 | € 16,512 | £ 8,666 | 27,944 | € 16,512 | £ 8,666
Principal Amount | SBA-guaranteed debentures |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 262,500 |  |  | 300,000 |  |
Principal Amount | 2022 Convertible Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 258,784 |  |  | 260,000 |  |
Fair Value |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 1,783,052 |  |  | 1,849,230 |  |
Fair Value | Unsecured Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 993,883 |  |  | 991,624 |  |
Fair Value | Credit Facility | Holdings Credit Facility | Revolving Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 266,747 |  |  | 298,435 |  |
Fair Value | Credit Facility | NMFC Credit Facility | Revolving Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 28,678 | € 16,288 | £ 8,560 | 26,812 | € 15,379 | £ 8,700
Fair Value | SBA-guaranteed debentures |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 231,596 |  |  | 270,548 |  |
Fair Value | 2022 Convertible Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | $ 262,148 |  |  | $ 261,811 |  |

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