Summary of Quarterly Results of Operations (Unaudited) (Details) (USD $) In Thousands, except Per 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
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 111,192	$ 112,157	$ 119,294	$ 121,174	$ 121,049	$ 118,482	$ 120,671	$ 122,738	$ 463,817	$ 482,940	$ 499,157
Interest expense	(38,960)	(39,358)	(40,569)	(40,921)	(43,427)	(41,950)	(38,478)	(40,158)	(159,808)	(164,013)	(171,670)
Net Interest Income	72,232	72,799	78,725	80,253	77,622	76,532	82,193	82,580	304,009	318,927	327,487
Gain on sales of MBS and U.S. Treasury securities, net	12,194	13,880	7,852	3,571	6,147	13,680	4,365	1,633	37,497	25,825	9,001
(Losses)/gains on TBA short positions					1,207	(8,724)	0	0	0	(7,517)	0
Unrealized net gains and net interest income from Linked Transactions	7,506	2,559	3,776	3,251	1,440	544	(295)	1,536	17,092	3,225	12,610
Other (loss)/income	502	54	56	(416)	54	55	55	55
Operating and other expense	(12,726)	(10,410)	(11,683)	(10,471)	(7,913)	(10,759)	(10,845)	(8,453)	(45,290)	(37,970)	(41,069)
Net Income	79,708	78,882	78,726	76,188	78,557	71,328	75,473	77,351	313,504	302,709	306,839
Preferred stock dividends	(3,750)	(3,750)	(3,750)	(3,750)	(3,750)	(3,750)	(4,210)	(2,040)	(15,000)	(13,750)	(8,160)
Issuance costs of redeemed preferred stock					0	0	(3,947)	0	0	(3,947)	0
Net Income Available to Common Stock and Participating Securities	$ 75,958	$ 75,132	$ 74,976	$ 72,438	$ 74,807	$ 67,578	$ 67,316	$ 75,311	$ 298,504	$ 285,012	$ 298,679
Earnings per Common Share - Basic and Diluted (in dollars per share)	$ 0.20	$ 0.20	$ 0.20	$ 0.20	$ 0.20	$ 0.19	$ 0.19	$ 0.21	$ 0.81	$ 0.78	$ 0.83