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Financial Highlights (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2025 USD ($) $ / shares shares	Jun. 30, 2024 USD ($) $ / shares	Jun. 30, 2025 USD ($) $ / shares shares	Jun. 30, 2024 USD ($) $ / shares shares	Jun. 30, 2025 EUR (€) shares	Jun. 30, 2025 GBP (£) shares	Dec. 31, 2024 USD ($) shares	Dec. 31, 2024 EUR (€) shares	Dec. 31, 2024 GBP (£) shares	Jun. 30, 2024 EUR (€) shares	Jun. 30, 2024 GBP (£) shares
Investment Company, Financial Highlights [Roll Forward]
Net asset value, beginning balance (in dollars per share) | $ / shares			$ 12.55	$ 12.87
Net investment income (in dollars per share) | $ / shares			0.64	0.72
Net realized and unrealized losses (in dollars per share) | $ / shares			(0.36)	(0.15)
Total net increase (in dollars per share) | $ / shares			0.28	0.57
Repurchase of common stock (in dollars per share) | $ / shares			0.02	0
Distributions declared to stockholders from net investment income (in dollars per share) | $ / shares			(0.64)	(0.70)
Net asset value, ending balance (in dollars per share) | $ / shares	$ 12.21	$ 12.74	12.21	12.74
Per share market value (in dollars per share) | $ / shares	$ 10.55	$ 12.24	$ 10.55	$ 12.24
Total return based on market value			(0.72%)	1.71%
Total return based on net asset value			2.44%	4.50%
Shares outstanding at end of period (in shares) | shares	106,926,713		106,926,713		106,926,713	106,926,713	107,851,415	107,851,415	107,851,415	107,851,415	107,851,415
Average weighted shares outstanding for the period (in shares) | shares			107,800,508	105,276,077
Average net assets for the period			$ 1,347,006	$ 1,349,273
Ratio to average net assets:
Net investment income			10.33%	11.26%
Total expenses, before waivers/reimbursements			15.62%	16.53%
Total expenses, net of waivers/reimbursements			14.96%	16.26%
Asset coverage ratio	185.56%		185.56%		185.56%	185.56%				182.80%	182.80%
Portfolio turnover			7.94%	14.94%
Unsecured Notes
Ratio to average net assets:
Average debt outstanding	$ 990,000	$ 690,000	$ 990,000	$ 661,305
SBA-guaranteed debentures
Ratio to average net assets:
Average debt outstanding	262,500	300,000	274,724	300,000
2022 Convertible Notes
Ratio to average net assets:
Average debt outstanding	258,784	260,000	259,147	260,000
Holdings Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	246,063		246,063				$ 294,363
Holdings Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	246,063		246,063				294,363
Holdings Credit Facility | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding	251,167	328,651	280,737	343,570
NMFC Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	31,372		31,372				27,944
NMFC Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	31,372		31,372		€ 16,512	£ 8,666	$ 27,944	€ 16,512	£ 8,666	€ 17,400	£ 22,850
NMFC Credit Facility | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding	$ 30,315	63,131	29,314	49,430
DB Credit Facility | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding		182,000	0	183,547
NMNLC Credit Facility II | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding		$ 2,505	$ 0	$ 2,438

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