Financial Highlights (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Per share data:
Net asset value per share, beginning balance (in dollars per share)	$ 11.05	$ 11.36	$ 10.99	$ 11.66	$ 10.98	$ 13.43	$ 15.13	$ 15.23	$ 16.11	$ 16.10
Net investment income (in dollars per share)	1.19	1.12	0.90	0.64	0.61	0	1.55	1.62	2.16	2.08
Net realized gain (loss) on investments / foreign currency transactions / forward currency contracts (in dollars per share)	(0.59)	0.16	(0.05)	(0.79)	(0.08)	(3.17)	(1.11)	0.05	(0.83)	0.46
Net unrealized appreciation (depreciation) on investments / CSAs / foreign currency transactions / forward currency contracts (in dollars per share)	0.59	(1.20)	0.34	0.38	0.64	1.08	(1.04)	(0.72)	0.17	(1.48)
Total increase (decrease) from investment operations (in dollars per share)	1.19	0.08	1.19	0.23	1.17	(2.09)	(0.60)	0.95	1.50	1.06
Dividends/distributions paid to stockholders from net investment income (in dollars per share)	(1.02)	(0.95)	(0.79)	(0.65)	(0.54)	(0.41)	(1.65)	(1.89)	(2.11)	(1.88)
Dividends paid to stockholders from realized gains (in dollars per share)						0	0	0	(0.25)	(0.68)
Tax return of capital to stockholders (in dollars per share)	0	0	(0.03)	0	0	(0.02)	0	0	0	0
Total dividends and distributions paid (in dollars per share)	(1.02)	(0.95)	(0.82)	(0.65)	(0.54)	(0.43)	(1.65)	(1.89)	(2.36)	(2.56)
Common stock offerings (in dollars per share)				(0.63)
Common stock offerings (in dollars per share)	0	0	0		0	0	0.61	0.72	0	1.49
Purchase of shares in tender offer (in dollars per share)						0.13	0	0	0	0
Stock-based compensation (in dollars per share)						0.17	(0.01)	0.09	0.01	0
Shares issued pursuant to Dividend Reinvestment Plan (in dollars per share)						0	0.01	0.04	0.03	0.04
Sierra Merger (in dollars per share)	0	0.10	0	0	0
Deemed contribution -CSA (in dollars per share)	0	0.40	0	0.28	0
Deemed contribution - Barings LLC (in dollars per share)	0	0	0	0.07	0
Purchases of shares in share repurchase plan (in dollars per share)	0.06	0.06	0	0.05	0.07
Loss on extinguishment of debt (in dollars per share)	0	0	0	0.06	0.01	(0.21)	0	0	(0.04)	0
Benefit from (provision for) taxes (in dollars per share)	0	0	0	0	0.01	0.02	(0.02)	(0.01)	(0.01)	(0.10)
Other (in dollars per share)	0	0	0	(0.04)	0	(0.04)	(0.04)	0	(0.01)	0.08
Net asset value per share, ending balance (in dollars per share)	11.28	11.05	11.36	10.99	11.66	10.98	13.43	15.13	15.23	16.11
Market value at end of period (in dollars per share)	$ 8.58	$ 8.15	$ 11.02	$ 9.20	$ 10.28	$ 9.01	$ 9.49	$ 18.34	$ 19.11	$ 20.29
Shares outstanding at end of period (in shares)	106,067,070	107,916,166	65,316,085	65,316,085	48,950,803	51,284,064	47,740,832	40,401,292	33,375,126	32,950,288
Net assets at end of period	$ 1,196,559	$ 1,192,329	$ 741,931	$ 717,805	$ 570,875	$ 562,967	$ 641,275	$ 611,156	$ 508,368	$ 530,827
Average net assets	$ 1,211,289	$ 1,184,591	$ 739,250	$ 517,740	$ 579,199	$ 628,155	$ 667,188	$ 556,549	$ 524,580	$ 482,679
Ratio of total expenses, prior to waiver of base management fee, including loss on extinguishment of debt and benefit from (provision for) taxes, to average net assets	13.30%	8.80%	10.33%	8.33%	7.90%	14.54%	7.74%	9.93%	9.81%	9.45%
Ratio of total expenses, net of base management fee waived, including loss on extinguishment of debt and benefit from (provision for) taxes, to average net assets						14.31%	7.74%	9.93%	9.81%	9.45%
Ratio of net investment income to average net assets	10.55%	9.76%	7.98%	5.99%	5.27%	(0.01%)	10.83%	10.58%	13.65%	12.85%
Portfolio turnover ratio	22.93%	43.07%	68.63%	67.80%	113.99%	228.49%	37.02%	24.61%	37.62%	29.21%
Total return	18.83%	(18.35%)	29.34%	(2.17%)	20.27%	18.18%	(42.15%)	5.86%	5.82%	(17.36%)