Quarterly Results of Operations (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2014	Sep. 30, 2014	Jun. 30, 2014	Mar. 31, 2014	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
Interest income:
Mortgage-backed securities	$ 87,702	$ 80,969	$ 89,458	$ 96,307	$ 104,312	$ 107,040	$ 115,115	$ 124,241	$ 354,436	$ 450,708	$ 504,800
Mortgage loans held for investment	35								35
Short-term cash investments	324	330	347	282	456	303	359	442	1,283	1,560	1,508
Total interest income	88,061	81,299	89,805	96,589	104,768	107,343	115,474	124,683
Interest expense	26,966	31,950	35,128	38,451	40,754	51,599	52,079	53,277	132,495	197,709	197,064
Net interest margin	61,095	49,349	54,677	58,138	64,014	55,744	63,395	71,406	223,259	254,559	309,244
Operating expenses:
Management fee	4,112	4,122	4,144	4,154	4,224	4,522	4,714	4,720	16,532	18,180	17,420
Share-based compensation	1,020	890	842	860	701	637	627	629	3,612	2,594	1,920
General and administrative	3,941	2,113	2,324	2,147	2,583	1,538	1,602	1,369	10,525	7,092	5,006
Total operating expenses	9,073	7,125	7,310	7,161	7,508	6,697	6,943	6,718	30,669	27,866	24,346
Other income (loss):
Net realized gain (loss) on sale of mortgage-backed securities	2,107	237	(4,584)	7,436	(68,679)	(225,635)	8,802	2,500	5,196	(283,012)	64,347
Impairment of mortgage-backed securities						(8,102)				(8,102)
Gain on mortgage loans held for investment	8								8
Gain (loss) on derivative instruments, net	(79,988)	35,430	(55,260)	(41,615)	2,205	(77,456)	5,485	51	(141,433)	(69,715)
Total other income (loss)	(77,873)	35,667	(59,844)	(34,179)	(66,474)	(311,193)	14,287	2,551	(136,229)	(360,829)	64,347
Net income (loss)	(25,851)	77,891	(12,477)	16,798	(9,968)	(262,146)	70,739	67,239	56,361	(134,136)	349,245
Dividends on preferred stock	5,481	5,480	5,481	5,480	5,481	5,481	5,480	5,480	21,922	21,922	7,551
Net income (loss) available to common shareholders	$ (31,332)	$ 72,411	$ (17,958)	$ 11,318	$ (15,449)	$ (267,627)	$ 65,259	$ 61,759	$ 34,439	$ (156,058)	$ 341,694
Earnings (loss) per share - common stock, basic	$ (0.32)	$ 0.75	$ (0.19)	$ 0.12	$ (0.16)	$ (2.72)	$ 0.66	$ 0.62	$ 0.36	$ (1.59)	$ 3.67
Earnings (loss) per share - common stock, diluted	$ (0.32)	$ 0.75	$ (0.19)	$ 0.12	$ (0.16)	$ (2.72)	$ 0.66	$ 0.62	$ 0.36	$ (1.59)	$ 3.67
Dividends per share of common stock	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.50	$ 0.55	$ 0.70	$ 0.70	$ 2.00	$ 2.45	$ 3.30
Weighted average common shares outstanding, basic	96,728,821	96,563,132	96,515,599	96,606,081	97,389,614	98,318,205	98,830,054	98,827,587	96,603,634	98,337,362	93,185,520
Weighted average common shares outstanding, diluted	96,728,821	96,563,132	96,515,599	96,606,081	97,389,614	98,318,205	98,830,054	98,827,587	96,603,634	98,337,362	93,185,520