Quarterly Financial Information (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 32,014	$ 26,925	$ 25,922	$ 25,190	$ 24,124	$ 23,103	$ 24,856	$ 22,419	$ 110,051	$ 94,502	$ 91,898
Interest expense	19,053	14,751	14,175	11,595	10,056	9,889	8,714	7,519	59,574	36,178	25,231
Net interest income	12,961	12,174	11,747	13,595	14,068	13,214	16,142	14,900	50,477	58,324	66,667
Gain (loss) on derivative instruments, net	(81,981)	19,499	20,667	38,354	12,678	5,993	(15,802)	175	(3,461)	3,044	(5,606)
Loss on sale of investments, net	(5,428)	(1,726)	(12,444)	(3,775)	(902)	(5,211)	(3,709)	(1,708)	(23,373)	(11,530)	(4,238)
Fair value adjustments, net	16	(12)	(27)	(29)	(12)	(23)	(30)	(10)	(52)	(75)	(103)
Other operating income (expense), net	(566)	(409)	(339)	(253)	(50)	(109)	4	(46)	(1,567)	(201)	880
General and administrative expenses	3,492	3,964	4,006	3,643	3,843	3,599	4,097	4,280
Preferred stock dividends	2,963	2,956	2,942	2,940	2,910	2,808	2,641	2,435	11,801	10,794	9,185
Net income (loss) to common shareholders	(81,485)	22,630	12,710	41,367	19,053	7,503	(10,073)	6,616	(4,778)	23,099	33,914
Other comprehensive income (loss)	50,054	(21,914)	(9,760)	(45,462)	(14,584)	6,144	12,375	19,977	(27,082)	23,912	(19,841)
Comprehensive income (loss) to common shareholders	$ (31,431)	$ 716	$ 2,950	$ (4,095)	$ 4,469	$ 13,647	$ 2,302	$ 26,593	$ (31,860)	$ 47,011	$ 14,073
Net income (loss) per common share	$ (1.34)	$ 0.39	$ 0.23	$ 0.74	$ 0.36	$ 0.15	$ (0.20)	$ 0.13	$ (0.08)	$ 0.46	$ 0.69
Dividends declared per common share	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18	$ 0.18