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Financial Highlights (Details) $ / shares in Units, € in Thousands, £ in Thousands, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2023 USD ($) $ / shares shares	Sep. 30, 2022 USD ($) $ / shares shares	Sep. 30, 2023 USD ($) $ / shares shares	Sep. 30, 2022 USD ($) $ / shares shares	Sep. 30, 2023 GBP (£) shares	Sep. 30, 2023 EUR (€) shares	Dec. 31, 2022 USD ($) shares	Dec. 31, 2022 GBP (£) shares	Dec. 31, 2022 EUR (€) shares	Sep. 30, 2022 GBP (£) shares	Sep. 30, 2022 EUR (€) shares
Investment Company, Financial Highlights [Roll Forward]
Net asset value, beginning balance (in dollars per share) | $ / shares			$ 13.02	$ 13.49
Net investment income (in dollars per share) | $ / shares			1.17	0.94
Net realized and unrealized losses (in dollars per share) | $ / shares			(0.10)	(0.33)
Total net increase (in dollars per share) | $ / shares			1.07	0.61
Distributions declared to stockholders from net investment income (in dollars per share) | $ / shares			(1.03)	(0.90)
Net asset value, ending balance (in dollars per share) | $ / shares	$ 13.06	$ 13.20	13.06	13.20
Per share market value (in dollars per share) | $ / shares	$ 12.95	$ 11.53	$ 12.95	$ 11.53
Total return based on market value			13.49%	(9.64%)
Total return based on net asset value			8.39%	4.54%
Shares outstanding at end of period (in shares) | shares	101,439,649		101,439,649		101,439,649	101,439,649	100,937,026	100,937,026	100,937,026	100,937,026	100,937,026
Average weighted shares outstanding for the period - basic (in shares) | shares	100,954,898	100,830,075	100,943,049	99,955,432
Average net assets for the period			$ 1,322,558	$ 1,348,479
Ratio to average net assets:
Net investment income			11.95%	9.27%
Total expenses, before waivers/reimbursements			16.77%	12.85%
Total expenses, net of waivers/reimbursements			16.45%	12.50%
Asset coverage ratio	182.68%		182.68%		182.68%	182.68%				179.28%	179.28%
Portfolio turnover			4.36%	14.38%
Accretion effect (in dollars per share) | $ / shares	$ 0.00	$ 0.01	$ 0.00	$ 0.01
Unsecured Notes
Ratio to average net assets:
Average debt outstanding	$ 391,500	$ 539,870	$ 433,661	$ 525,456
SBA-guaranteed debentures
Ratio to average net assets:
Average debt outstanding	300,000	300,000	300,000	300,000
Convertible Notes
Ratio to average net assets:
Average debt outstanding	317,138	201,250	340,881	201,250
Holdings Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	569,563		569,563				$ 618,963
Holdings Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	569,563		569,563				618,963
Holdings Credit Facility | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding	563,470	607,585	603,154	580,699
DB Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	186,400		186,400				186,400
DB Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	186,400		186,400				186,400
DB Credit Facility | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding	186,487	199,442	186,429	217,817
NMFC Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	192,115		192,115				40,359
NMFC Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	192,115		192,115		£ 22,850	€ 700	40,359	£ 22,850	€ 700	£ 22,850	€ 700
NMFC Credit Facility | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding	141,764	171,850	104,227	156,303
NMNLC Credit Facility II
Ratio to average net assets:
Credit facility outstanding balance	2,858		2,858				3,785
NMNLC Credit Facility II | Revolving Credit Facility
Ratio to average net assets:
Credit facility outstanding balance	2,858		2,858				$ 3,785
NMNLC Credit Facility II | Credit Facility | Revolving Credit Facility
Ratio to average net assets:
Average debt outstanding	$ 2,091	$ 2,013	$ 2,480	$ 8,939

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