Financial Highlights - Summary of Financial Highlights for One Share Common Stock Outstanding (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
Investment Company, Financial Highlights [Abstract]
Net asset value, beginning of period	$ 16.48	$ 16.84	$ 17.16	$ 16.83	$ 16.25	$ 16.09	$ 15.95	$ 15.15	$ 15.53	$ 15.52
Net investment income	2.31	2.13	1.97	2.19	1.94	2.25	2	1.83	1.76	2.07
Net realized and unrealized gains (losses)	0.3	(0.75)	0.96	0.46	0.4	(0.39)	(0.14)	0.51	(0.58)	(0.33)
Total from operations	2.61	1.38	2.93	2.65	2.34	1.86	1.86	2.34	1.18	1.74
Issuance of common stock, net of offering costs	0.05	0.04	0.31	0.01	0.04	0.08	0.03	0.03		(0.2)
Settlement of 2022 Convertible Notes		0.08	0.03
Repurchase of common stock		(0.02)		0.01
Repurchase of Debt				(0.01)
Dividends declared from net investment income	(2.1)	(1.59)	(2.8)	(2.07)	(1.81)	(1.77)	(1.53)	(1.39)	(1.15)	(1.51)
Dividend declared from realized gains		(0.25)	(0.79)	(0.23)		(0.01)	(0.22)	(0.17)	(0.41)	(0.02)
Total increase/(decrease) in net assets	0.56	(0.36)	(0.32)	0.33	0.58	0.16	0.14	0.81	(0.38)	0.01
Net Asset Value, End of Period	17.04	16.48	16.84	17.16	16.83	16.25	16.09	15.95	15.15	15.53
Per share market value at end of period	$ 21.6	$ 17.8	$ 23.39	$ 20.75	$ 21.47	$ 18.09	$ 19.8	$ 18.68	$ 16.22	$ 16.82
Total return based on market value with reinvestment of dividends	35.68%	(15.78%)	32.80%	11.24%	30.57%	4.24%	15.75%	26.74%	5.75%	12.28%
Total return based on market value	33.14%	(16.03%)	30.02%	7.36%	28.69%	0.35%	15.36%	24.78%	5.71%	14.69%
Total return based on net asset value	16.09%	8.79%	19.06%	15.63%	14.71%	12.06%	11.87%	15.54%	7.62%	9.92%
Shares Outstanding, End of Period	87,829,499	81,389,287	75,771,542	67,684,209	66,524,591	65,412,817	60,247,201	59,716,205	54,163,960	53,797,358
Ratio of net expenses to average net assets	16.92%	11.05%	11.17%	11.10%	11.27%	11.32%	9.41%	9.39%	9.31%	7.56%
Ratio of net investment income to average net assets	13.75%	12.85%	11.67%	13.26%	11.73%	13.80%	12.49%	11.84%	11.35%	13.42%
Portfolio turnover	16.92%	26.67%	44.23%	41.88%	30.89%	44.57%	58.08%	37.40%	34.51%	53.16%
Net assets, end of period	$ 1,496,375	$ 1,341,569	$ 1,275,848	$ 1,161,315	$ 1,119,297	$ 1,063,202	$ 969,284	$ 952,212	$ 820,741	$ 835,405