Summarized Quarterly Results (unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Net Interest Income
Interest income	$ 34,878	$ 30,928	$ 30,055	$ 28,430	$ 35,764	$ 29,154	$ 29,220	$ 29,618	$ 124,291	$ 123,756	$ 152,704
Interest expense	16,866	12,363	10,407	8,737	9,039	7,685	7,727	7,979	48,373	32,430	27,605
Net Interest Income	18,012	18,565	19,648	19,693	26,725	21,469	21,493	21,639	75,918	91,326	125,099
Other Income (Loss)
Realized gain (loss) on sale of investments, net	(2)	1,830	(2,488)	21,258	(17,023)	1,439	(352)	(6,055)	20,598	(21,991)	8,279
Other than temporary impairment	(2,972)	(7,225)	(6,579)	(6,097)	(10,155)	(4,978)	(6,356)	(10,797)	(22,873)	(32,286)	(19,791)
Unrealized gain (loss), net	(6,730)	5,249	35,017	(5,140)	(64,678)	15,292	21,510	10,769
Gain (loss) on derivative instruments, net	19,327	7,217	(18,555)	(4,697)	32,479	6,121	(14,165)	(45,170)	3,292	(20,735)	(68,895)
Other, net	190	216	222	403	338	(60)	234	(332)
Other Income (Loss)	9,813	7,287	7,617	5,727	(59,039)	17,814	871	(51,585)	1,031	180	2,318
Expenses
Management fee to affiliate	1,941	1,853	1,830	2,476	2,503	2,604	2,588	2,753
Other operating expenses	564	702	736	417	236	188	183	438	2,419	1,045	2,014
Compensation expense	628	660	664	740	768	868	649	737	2,692	3,022	3,762
Professional fees	741	781	832	888	867	723	1,222	2,002	3,242	4,814	4,368
Other general and administrative expenses	332	244	404	345	691	379	419	428	1,325	1,917	1,465
Total general and administrative expenses	1,701	1,685	1,900	1,973	2,326	1,970	2,290	3,167	7,259	9,753	9,595
Total Expenses	4,206	4,240	4,466	4,866	5,065	4,762	5,061	6,358	17,778	21,246	22,483
Income (loss) before income taxes	23,619	21,612	22,799	20,554	(37,379)	34,521	17,303	(36,304)	88,584	(21,859)	(9,484)
Income tax provision (benefit)	2,215	(1,155)	2,115	312	917	2,239	0	0	3,487	3,156	0
Net income (loss)	$ 21,404	$ 22,767	$ 20,684	$ 20,242	$ (38,296)	$ 32,282	$ 17,303	$ (36,304)	$ 85,097	$ (25,015)	$ (9,484)
Net income (loss) per common share - basic (in dollars per share)	$ 0.51	$ 0.54	$ 0.49	$ 0.48	$ (0.92)	$ 0.77	$ 0.41	$ (0.88)	$ 2.03	$ (0.61)	$ (0.25)
Net income (loss) per common share - diluted (in dollars per share)	$ 0.51	$ 0.54	$ 0.49	$ 0.48	$ (0.92)	$ 0.77	$ 0.41	$ (0.88)	$ 2.03	$ (0.61)	$ (0.25)