Selected Quarterly Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Selected Quarterly Information (Unaudited) [Abstract]
Interest income	$ 22,858	$ 21,135	$ 22,816	$ 25,089	$ 25,522	$ 26,096	$ 24,527	$ 24,099	$ 91,898	$ 100,244	$ 105,644
Interest expense	6,753	6,068	6,100	6,310	5,833	5,859	5,542	5,371	25,231	22,605	25,915
Net interest income	16,105	15,067	16,716	18,779	19,689	20,237	18,985	18,728	66,667	77,639	79,729
(Loss) gain on derivatives instruments, net	56,546	2,409	(16,297)	(48,264)	17,854	(52,749)	17,090	(25,323)	(5,606)	(43,128)	(53,393)
(Loss) gain on sale of investments, net	0	0	(297)	(3,941)	(908)	113	(1,491)	1,308	(4,238)	(978)	16,223
Fair value adjustments and other income (expense) amounts, net	(1)	579	318	87	174	(199)	632	72
General and administrative expenses	(3,589)	(3,355)	(3,671)	(4,092)	(4,278)	(4,379)	(4,754)	(4,257)
Preferred stock dividends	(2,303)	(2,294)	(2,294)	(2,294)	(2,294)	(2,294)	(2,294)	(2,294)	(9,185)	(9,176)	(9,176)
Net Income (Loss) Available to Common Stockholders, Basic	66,758	12,406	(5,525)	(39,725)	30,237	(39,271)	28,168	(11,766)	33,914	7,368	18,630
Other comprehensive income (loss)	(85,186)	670	22,947	41,728	(44,877)	27,418	(39,679)	23,054	(19,841)	(34,084)	55,132
Comprehensive income (loss) to common shareholders	$ (18,428)	$ 13,076	$ 17,422	$ 2,003	$ (14,640)	$ (11,853)	$ (11,511)	$ 11,288	$ 14,073	$ (26,716)	$ 73,762
Net income per common share-basic and diluted	$ 1.36	$ 0.25	$ (0.11)	$ (0.81)	$ 0.61	$ (0.74)	$ 0.52	$ (0.21)	$ 0.69	$ 0.14	$ 0.34
Dividends declared per common share	$ 0.21	$ 0.21	$ 0.21	$ 0.21	$ 0.24	$ 0.24	$ 0.24	$ 0.24