Quarterly Results of Operations (Details) (USD $) In Thousands, except Share data, unless otherwise specified	3 Months Ended								12 Months Ended
	Dec. 31, 2013	Sep. 30, 2013	Jun. 30, 2013	Mar. 31, 2013	Dec. 31, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Interest income:
Interest income on mortgage-backed securities	$ 104,312	$ 107,040	$ 115,115	$ 124,241	$ 131,381	$ 131,940	$ 128,720	$ 112,759	$ 450,708	$ 504,800	$ 424,713
Interest income on short-term cash investments	456	303	359	442	347	387	441	333	1,560	1,508	1,407
Total interest income	104,768	107,343	115,474	124,683	131,728	132,327	129,161	113,092	452,268	506,308	426,120
Interest expense	40,754	51,599	52,079	53,277	57,019	52,767	46,169	41,109	197,709	197,064	144,662
Net interest margin	64,014	55,744	63,395	71,406	74,709	79,560	82,992	71,983	254,559	309,244	281,458
Operating expenses:
Management fee	4,224	4,522	4,714	4,720	4,709	4,557	4,312	3,842	18,180	17,420	13,787
Share based compensation	701	637	627	629	619	447	425	429	2,594	1,920	1,150
General and administrative	2,583	1,538	1,602	1,369	1,737	1,040	1,316	913	7,092	5,006	2,724
Total operating expenses	7,508	6,697	6,943	6,718	7,065	6,044	6,053	5,184	27,866	24,346	17,661
Other income (loss):
Net realized gain (loss) on sale of mortgage-backed securities	(68,679)	(225,635)	8,802	2,500	39,103	10,534	12,205	2,505	(283,012)	64,347	20,576
Impairment of mortgage-backed securities		(8,102)
Gain (loss) on derivative instruments, net	2,205	(77,456)	5,485	51
Total other income (loss)	(66,474)	(311,193)	14,287	2,551					(360,829)	64,347	20,576
Net income (loss)	(9,968)	(262,146)	70,739	67,239	106,747	84,050	89,144	69,304	(134,136)	349,245	284,373
Dividends on preferred stock	5,481	5,481	5,480	5,480	5,481	2,070	0	0	21,922	7,551
Net income (loss) available to common shareholders	$ (15,449)	$ (267,627)	$ 65,259	$ 61,759	$ 101,266	$ 81,980	$ 89,144	$ 69,304	$ (156,058)	$ 341,694	$ 284,373
Earnings (loss) per share - common stock, basic	$ (0.16)	$ (2.72)	$ 0.66	$ 0.62	$ 1.02	$ 0.83	$ 0.91	$ 0.89	$ (1.59)	$ 3.67	$ 3.97
Earnings (loss) per share - common stock, diluted	$ (0.16)	$ (2.72)	$ 0.66	$ 0.62	$ 1.02	$ 0.83	$ 0.91	$ 0.89	$ (1.59)	$ 3.67	$ 3.97
Dividends per share	$ 0.50	$ 0.55	$ 0.70	$ 0.70	$ 0.70	$ 0.80	$ 0.90	$ 0.90
Weighted average shares outstanding	97,389,614	98,318,205	98,830,054	98,827,587	98,811,996	98,233,589	97,969,074	77,610,117