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Financial Highlights (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2025 USD ($) $ / shares shares	Sep. 30, 2024 USD ($) $ / shares	Sep. 30, 2025 USD ($) $ / shares shares	Sep. 30, 2024 USD ($) $ / shares shares	Sep. 30, 2025 EUR (€) shares	Sep. 30, 2025 GBP (£) shares	Dec. 31, 2024 USD ($) shares	Dec. 31, 2024 EUR (€) shares	Dec. 31, 2024 GBP (£) shares	Sep. 30, 2024 EUR (€) shares	Sep. 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			0.96	1.05
Net realized and unrealized losses (in dollars per share) | $ / shares			(0.56)	(0.26)
Total net increase (in dollars per share) | $ / shares			0.40	0.79
Repurchase of common stock (in dollars per share) | $ / shares			0.07	0
Distributions declared to stockholders from net investment income (in dollars per share) | $ / shares			(0.96)	(1.04)
Net asset value, ending balance (in dollars per share) | $ / shares	$ 12.06	$ 12.62	12.06	12.62
Per share market value (in dollars per share) | $ / shares	$ 9.64	$ 11.98	$ 9.64	$ 11.98
Total return based on market value			(6.24%)	2.40%
Total return based on net asset value			3.86%	6.32%
Shares outstanding at end of period (in shares) | shares	104,189,336		104,189,336		104,189,336	104,189,336	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			107,199,318	106,140,789
Average net assets for the period			$ 1,329,882	$ 1,357,569
Ratio to average net assets:
Net investment income			10.35%	10.93%
Total expenses, before waivers/reimbursements			15.62%	16.90%
Total expenses, net of waivers/reimbursements			14.72%	16.63%
Asset coverage ratio	179.09%		179.09%		179.09%	179.09%				179.18%	179.18%
Portfolio turnover			12.18%	17.99%
Accretive effect of common stock issuances per share (in dollars per share) | $ / shares			$ 0.00	$ (0.02)
Holdings Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	$ 308,063		$ 308,063				$ 294,363
NMFC Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	31,032		31,032				27,944
Revolving Credit Facility | Holdings Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	308,063		308,063				294,363
Revolving Credit Facility | NMFC Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	31,032		31,032		€ 16,512	£ 8,666	$ 27,944	€ 16,512	£ 8,666	€ 17,400	£ 26,650
Unsecured Notes
Ratio to average net assets:
Average debt outstanding	990,000	$ 706,304	990,000	$ 676,414
Line of Credit | Revolving Credit Facility | Holdings Credit Facility
Ratio to average net assets:
Average debt outstanding	293,932	405,661	285,184	364,418
Line of Credit | Revolving Credit Facility | NMFC Credit Facility
Ratio to average net assets:
Average debt outstanding	30,991	140,385	29,879	79,969
Line of Credit | Revolving Credit Facility | DB Credit Facility
Ratio to average net assets:
Average debt outstanding		180,022	0	182,364
Line of Credit | Revolving Credit Facility | NMNLC Credit Facility II
Ratio to average net assets:
Average debt outstanding		2,151	0	2,341
SBA-guaranteed debentures
Ratio to average net assets:
Average debt outstanding	240,882	300,000	263,319	300,000
2022 Convertible Notes
Ratio to average net assets:
Average debt outstanding	$ 258,777	$ 260,000	$ 259,022	$ 260,000

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