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Financial Highlights - Senior Securities (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2023 USD ($) $ / shares	Dec. 31, 2022 USD ($) $ / shares	Dec. 31, 2021 USD ($) $ / 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, 2014 USD ($) $ / shares	Dec. 31, 2023 GBP (£)	Dec. 31, 2023 EUR (€)	Nov. 13, 2023	Dec. 31, 2022 GBP (£)	Dec. 31, 2022 EUR (€)	Dec. 31, 2021 GBP (£)	Dec. 31, 2021 EUR (€)	Sep. 25, 2018
Debt Instrument [Line Items]
SBA-guaranteed debentures outstanding	$ 300,000	$ 300,000	$ 300,000	$ 300,000	$ 225,000	$ 165,000	$ 150,000	$ 121,700	$ 117,700	$ 37,500
SBA-guaranteed debentures, coverage per unit (in dollars per share) \| $ / shares	$ 1,730	$ 1,658	$ 1,686	$ 1,673	$ 1,655	$ 1,718	$ 2,169	$ 2,320	$ 2,128	$ 2,196
5.75% Unsecured Notes \| Unsecured Notes
Debt Instrument [Line Items]
Stated interest rate																		5.75%
8.25% Notes due 2028 \| Unsecured Notes
Debt Instrument [Line Items]
Stated interest rate													8.25%					8.25%
NMFC Credit Facility
Debt Instrument [Line Items]
Credit facility outstanding balance	$ 36,813	$ 40,359
NMFC Credit Facility \| Revolving Credit Facility
Debt Instrument [Line Items]
Credit facility outstanding balance			$ 127,192								£ 22,900	€ 700		£ 22,900	€ 700	£ 16,400	€ 0