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Financial Highlights - Summary (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands	12 Months Ended
	Dec. 31, 2024 USD ($) $ / shares shares	Dec. 31, 2023 USD ($) $ / shares shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 USD ($) $ / shares shares	Dec. 31, 2020 USD ($) $ / shares shares	Dec. 31, 2024 EUR (€) shares	Dec. 31, 2024 GBP (£) shares	Dec. 31, 2024 USD ($) $ / shares shares	Sep. 26, 2024	Feb. 01, 2024	Dec. 31, 2023 EUR (€) shares	Dec. 31, 2023 GBP (£) shares	Dec. 31, 2023 USD ($) $ / shares shares	Nov. 13, 2023	Dec. 31, 2022 EUR (€) shares	Dec. 31, 2022 GBP (£) shares	Dec. 31, 2022 USD ($) $ / shares shares	Dec. 31, 2021 EUR (€) shares	Dec. 31, 2021 GBP (£) shares	Sep. 25, 2018
Investment Company, Financial Highlights [Roll Forward]
Net asset value, beginning balance (in dollars per share) | $ / shares	$ 12.87	$ 13.02	$ 13.49	$ 12.62	$ 13.26
Net investment income (in dollars per share) | $ / shares	1.37	1.57	1.18	1.21	1.20
Net realized and unrealized (losses) gains (in dollars per share) | $ / shares	(0.32)	(0.23)	(0.43)	0.86	(0.60)
Total net increase (in dollars per share) | $ / shares	1.05	1.34	0.75	2.07	0.60
Distributions declared to stockholders from net investment income (in dollars per share) | $ / shares	(1.37)	(1.49)	(1.22)	(1.20)	(1.24)
Net asset value, ending balance (in dollars per share) | $ / shares	$ 12.55	$ 12.87	$ 13.02	13.49	12.62
Per share market value (in dollars per share) | $ / shares				$ 13.70	$ 11.36			$ 11.26					$ 12.72				$ 12.37
Total return based on market value	(0.96%)	15.48%	(0.56%)	31.91%	(5.24%)
Total return based on net asset value	8.47%	10.64%	5.71%	16.97%	5.52%
Shares outstanding at end of period (in shares) | shares					96,827,342	107,851,415	107,851,415	107,851,415			102,558,859	102,558,859	102,558,859		100,937,026	100,937,026	100,937,026	97,907,441	97,907,441
Average weighted shares outstanding for the period (in shares) | shares	106,570,782	101,118,302	100,202,847	96,952,959	96,827,342
Average net assets for the period	$ 1,358,484	$ 1,323,778	$ 1,344,266	$ 1,261,338	$ 1,168,043
Ratio to average net assets:
Net investment income	10.71%	12.00%	8.82%	9.32%	10.05%
Total expenses, before waivers/reimbursements	16.89%	16.54%	13.35%	13.11%	14.56%
Total expenses, net of waivers/reimbursements	16.61%	16.23%	13.01%	12.05%	13.39%
Asset coverage ratio					180.68%	186.70%	186.70%	186.70%			187.54%	187.54%	187.54%		177.42%	177.42%	177.42%	181.21%	181.21%
Portfolio turnover	24.84%	8.87%	18.01%	35.33%	15.43%
Effect of common stock issuance (in dollars per share) | $ / shares	$ (0.02)	$ 0.00	$ 0.01	$ (0.01)	$ 0.00
Unsecured Notes
Ratio to average net assets:
Average debt outstanding	$ 755,239	$ 438,473	$ 526,829	$ 516,611	$ 453,250
SBA-guaranteed debentures
Ratio to average net assets:
Average debt outstanding	300,000	300,000	300,000	300,000	285,852
Convertible Notes
Ratio to average net assets:
Average debt outstanding	260,000	320,494	228,806	201,250	201,250
Holdings Credit Facility
Ratio to average net assets:
Credit facility outstanding balance								$ 294,363					$ 515,063				$ 618,963
Holdings Credit Facility | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding	369,292	577,759	581,367	478,016	526,645
DB Credit Facility
Ratio to average net assets:
Credit facility outstanding balance								0					186,400
DB Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Credit facility outstanding balance																	186,400
DB Credit Facility | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding	182,364	186,422	209,898	209,307	233,649
NMFC Credit Facility
Ratio to average net assets:
Credit facility outstanding balance								27,944					36,813
NMFC Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Credit facility outstanding balance						€ 16,512	£ 8,700				€ 700	£ 22,900			€ 700	£ 22,900	40,359	€ 0	£ 16,400
NMFC Credit Facility | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding	74,245	103,244	133,053	132,685	155,497
NMNLC Credit Facility II
Ratio to average net assets:
Credit facility outstanding balance								0					2,853
NMNLC Credit Facility II | Revolving Credit Facility
Ratio to average net assets:
Credit facility outstanding balance								$ 0					$ 2,853				$ 3,785
NMNLC Credit Facility II | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding	$ 2,362	$ 2,506	$ 7,195	$ 3,501	$ 0
6.875% Notes due 2029 | Unsecured Notes
Ratio to average net assets:
Stated interest rate						6.875%	6.875%	6.875%		6.875%
6.200% Notes due 2027 | Unsecured Notes
Ratio to average net assets:
Stated interest rate						6.20%	6.20%	6.20%	6.20%
8.25% Notes due 2028
Ratio to average net assets:
Stated interest rate																				8.25%
8.25% Notes due 2028 | Unsecured Notes
Ratio to average net assets:
Stated interest rate						8.25%	8.25%	8.25%						8.25%						8.25%

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