Fair Value - Schedule of Principal Amounts and Fair Values of the Company’s Borrowings (Details) € in Thousands, £ in Thousands, $ in Thousands	Sep. 30, 2026 USD ($)	Sep. 30, 2026 EUR (€)	Sep. 30, 2026 GBP (£)	Dec. 31, 2025 USD ($)	Dec. 31, 2025 EUR (€)	Dec. 31, 2025 GBP (£)
Principal Amount
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Borrowings	$ 1,331,658			$ 1,687,342
Principal Amount \| Unsecured Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Borrowings	790,000			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	391,446			420,063
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	30,212	€ 16,512	£ 8,666	81,074	€ 16,512	£ 8,666
Principal Amount \| SBA-guaranteed debentures
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Borrowings	120,000			196,205
Fair Value
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Borrowings	1,308,693			1,678,575
Fair Value \| Unsecured Notes
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Borrowings	790,775			1,002,356
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	389,677			419,981
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	30,278	€ 16,547	£ 8,686	81,097	€ 16,479	£ 8,651
Fair Value \| SBA-guaranteed debentures
Fair Value Disclosure, Asset and Liability, Not Measured at Fair Value [Line Items]
Borrowings	$ 97,963			$ 175,141