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Fair Value - Schedule of Principal Amounts and Fair Values of the Company’s Borrowings (Details) € in Thousands, £ in Thousands, $ in Thousands | Sep. 30, 2025 USD ($) | Sep. 30, 2025 EUR (€) | Sep. 30, 2025 GBP (£) | Dec. 31, 2024 USD ($) | Dec. 31, 2024 EUR (€) | Dec. 31, 2024 GBP (£)
--- | --- | --- | --- | --- | --- | ---
Principal Amount |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | $ 1,784,077 |  |  | $ 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 | Line of Credit | Revolving Credit Facility | Holdings Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 308,063 |  |  | 294,363 |  |
Principal Amount | Line of Credit | Revolving Credit Facility | NMFC Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 31,032 | € 16,512 | £ 8,666 | 27,944 | € 16,512 | £ 8,666
Principal Amount | 2022 Convertible Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 258,777 |  |  | 260,000 |  |
Principal Amount | SBA-guaranteed debentures |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 196,205 |  |  | 300,000 |  |
Fair Value |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 1,775,934 |  |  | 1,849,230 |  |
Fair Value | Unsecured Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 1,004,496 |  |  | 991,624 |  |
Fair Value | Line of Credit | Revolving Credit Facility | Holdings Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 307,999 |  |  | 298,435 |  |
Fair Value | Line of Credit | Revolving Credit Facility | NMFC Credit Facility |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 30,969 | € 16,477 | £ 8,650 | 26,812 | € 15,379 | £ 8,700
Fair Value | 2022 Convertible Notes |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | 259,036 |  |  | 261,811 |  |
Fair Value | SBA-guaranteed debentures |  |  |  |  |  |
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items] |  |  |  |  |  |
Borrowings | $ 173,434 |  |  | $ 270,548 |  |

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