Selected Quarterly Information (Unaudited) (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2015	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Selected Quarterly Information (Unaudited) [Abstract]
Interest income	$ 25,522	$ 26,096	$ 24,527	$ 24,099	$ 24,286	$ 26,000	$ 27,718	$ 27,640	$ 100,244	$ 105,644	$ 127,132
Interest Expense	5,833	5,859	5,542	5,371	5,652	6,058	6,572	7,633	22,605	25,915	39,028
Net interest income	19,689	20,237	18,985	18,728	18,634	19,942	21,146	20,007	77,639	79,729	88,104
(Loss) gain on derivative instruments, net	(17,854)	52,749	(17,090)	25,323	21,739	(4,842)	23,074	13,422	43,128	53,393	10,076
Gain (loss) on sale of investments, net	908	(113)	1,491	(1,308)	(10,950)	(9,057)	477	3,307	978	(16,223)	(3,354)
Fair value adjustments and other income (expense) amounts, net	174	(199)	632	72	(17)	939	225	107
Other General and Administrative Expense	(4,278)	(4,379)	(4,754)	(4,257)	(4,155)	(3,914)	(3,819)	(4,119)
Preferred stock dividends	(2,294)	(2,294)	(2,294)	(2,294)	(2,294)	(2,294)	(2,294)	(2,294)	(9,176)	(9,176)	(7,902)
Net income to common shareholders	30,237	(39,271)	28,168	(11,766)	1,379	28,572	(8,293)	(3,028)	7,368	18,630	60,167
Other comprehensive income (loss)	(44,877)	27,418	(39,679)	23,054	4,855	(14,482)	35,199	29,560	(34,084)	55,132	(86,327)
Comprehensive (loss) income to common shareholders	$ (14,640)	$ (11,853)	$ (11,511)	$ 11,288	$ 6,234	$ 14,090	$ 26,906	$ 26,532	$ (26,716)	$ 73,762	$ (26,160)
Net (loss) income per common share	$ 0.61	$ (0.74)	$ 0.52	$ (0.21)	$ 0.03	$ 0.52	$ (0.15)	$ (0.06)	$ 0.14	$ 0.34	$ 1.10
Dividends declared per common share	$ 0.24	$ 0.24	$ 0.24	$ 0.24	$ 0.25	$ 0.25	$ 0.25	$ 0.25