Summarized Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Net Interest Income
Interest income	$ 55,761	$ 55,652	$ 53,818	$ 52,033	$ 58,020	$ 54,461	$ 57,154	$ 39,727	$ 217,264	$ 209,362	$ 124,291
Interest expense (includes $5,674, $13,739 and $981 on securitized debt held by affiliates, respectively)	36,834	39,082	37,958	36,400	40,892	38,517	38,134	20,697	150,274	138,240	48,373
Net Interest Income	18,927	16,570	15,860	15,633	17,128	15,944	19,020	19,030	66,990	71,122	75,918
Realized gain (loss) on sale of investments, net	11,992	21,399	(8)	(5,105)	(33,995)	(24,229)	(5,608)	575	28,278	(63,257)	20,598
Other Income (Loss)
Other than temporary impairment	(2,228)	(1,819)	(3,295)	(1,232)	(2,757)	(2,533)	(2,974)	(2,916)
Unrealized gain (loss), net					62,855	13,128	(31,693)	(68,961)	107,529	(24,671)	28,396
Unrealized gain (loss), net	(52,896)	35,030	74,614	50,781
Gain (loss) on derivative instruments, net	42,007	(47,056)	(71,530)	(27,148)	(54,728)	24,625	28,490	79,582	(103,727)	77,969	3,292
Other, net	518	918	532	236	(89)	(2)	(145)	47
Other Income (Loss)	(607)	8,472	313	17,532	(28,714)	10,989	(11,930)	8,327	2,204	(189)	1,031
Expenses
Management fee to affiliate	1,987	1,800	1,832	1,735	1,950	2,284	2,259	2,180	7,354	8,673	8,100
Other operating expenses	1,079	1,589	1,253	1,598	1,943	1,609	1,555	969	5,519	6,076	2,419
Compensation expense	671	671	705	544	552	552	572	510	2,591	2,186	2,692
Professional fees	1,031	973	761	1,215	1,121	1,065	818	1,295	3,980	4,299	3,242
Other general and administrative expenses	441	344	530	185	349	335	397	361	1,500	1,442	1,325
Total general and administrative expenses	2,143	1,988	1,996	1,944	2,022	1,952	1,787	2,166	8,071	7,927	7,259
Total Expenses	5,209	5,377	5,081	5,277	5,915	5,845	5,601	5,315	20,944	22,676	17,778
Income before income taxes	13,111	19,665	11,092	27,888	(17,501)	21,088	1,489	22,042	71,756	27,118	88,584
Income tax provision (benefit)	622	(55)	478	12	154	206	36	313	1,057	709	3,487
Net income (loss)	$ 12,489	$ 19,720	$ 10,614	$ 27,876	$ (17,655)	$ 20,882	$ 1,453	$ 21,729	$ 70,699	$ 26,409	$ 85,097
Net income per Common Share — Basic (in dollars per share)	$ 0.23	$ 0.37	$ 0.21	$ 0.58	$ (0.37)	$ 0.50	$ 0.03	$ 0.52	$ 1.37	$ 0.61	$ 2.03
Net income per Common Share — Diluted (in dollars per share)	$ 0.23	$ 0.37	$ 0.21	$ 0.58	$ (0.37)	$ 0.50	$ 0.03	$ 0.52	$ 1.37	$ 0.61	$ 2.03