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Financial Highlights - Senior Securities (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands	12 Months Ended
	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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