Selected Quarterly Information (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
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 24,124	$ 23,103	$ 24,856	$ 22,419	$ 22,858	$ 21,135	$ 22,816	$ 25,089	$ 94,502	$ 91,898	$ 100,244
Interest expense	10,056	9,889	8,714	7,519	6,753	6,068	6,100	6,310	36,178	25,231	22,605
Net interest income	14,068	13,214	16,142	14,900	16,105	15,067	16,716	18,779	58,324	66,667	77,639
Gain (loss) on derivative instruments, net	12,678	5,993	(15,802)	175	56,546	2,409	(16,297)	(48,264)	3,044	(5,606)	(43,128)
Loss on sale of investments, net	(902)	(5,211)	(3,709)	(1,708)	0	0	(297)	(3,941)	(11,530)	(4,238)	(978)
Fair value adjustments and other income (expense) amounts, net	(38)	(86)	34	(36)	(1)	579	318	87
General and administrative expenses	3,843	3,599	4,097	4,280	3,589	3,355	3,671	4,092
Preferred stock dividends	2,910	2,808	2,641	2,435	2,303	2,294	2,294	2,294	10,794	9,185	9,176
Net income (loss) to common shareholders	19,053	7,503	(10,073)	6,616	66,758	12,406	(5,525)	(39,725)	23,099	33,914	7,368
Other comprehensive income	(14,584)	6,144	12,375	19,977	(85,186)	670	22,947	41,728	23,912	(19,841)	(34,084)
Comprehensive income (loss) to common shareholders	$ 4,469	$ 13,647	$ 2,302	$ 26,593	$ (18,428)	$ 13,076	$ 17,422	$ 2,003	$ 47,011	$ 14,073	$ (26,716)
Net income per common share-basic and diluted	$ 0.36	$ 0.15	$ (0.20)	$ 0.13	$ 1.36	$ 0.25	$ (0.11)	$ (0.81)	$ 0.46	$ 0.69	$ 0.14
Dividends declared per common share	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.21	$ 0.21	$ 0.21	$ 0.21