Summary of Quarterly Results of Operations (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
Quarterly Financial Information Disclosure [Abstract]
Interest income	$ 118,499	$ 119,706	$ 123,995	$ 129,943	$ 111,192	$ 112,157	$ 119,294	$ 121,174	$ 492,143	$ 463,817	$ 482,940
Interest expense	(46,456)	(43,703)	(42,849)	(43,940)	(38,960)	(39,358)	(40,569)	(40,921)	(176,948)	(159,808)	(164,013)
Net Interest Income	72,043	76,003	81,146	86,003	72,232	72,799	78,725	80,253	315,195	304,009	318,927
Net Impairment Losses Recognized in Earnings	0	0	(298)	(407)					(705)	0	0
Gain on sales of MBS and U.S. Treasury securities, net	9,652	11,196	7,617	6,435	12,194	13,880	7,852	3,571	34,900	37,497	25,825
Unrealized net gains and net interest income from Linked Transactions					7,506	2,559	3,776	3,251	0	17,092	3,225
Net gain on residential whole loans held at fair value	6,899	5,565	3,224	2,034	116	0	0	0	17,722	116	0
Other income/(loss)	(831)	(259)	(678)	311	386	54	56	(416)
Operating and other expense	(14,292)	(12,995)	(12,940)	(12,202)	(12,726)	(10,410)	(11,683)	(10,471)	(52,429)	(45,290)	(37,970)
Net Income	73,471	79,510	78,071	82,174	79,708	78,882	78,726	76,188	313,226	313,504	302,709
Preferred stock dividends	(3,750)	(3,750)	(3,750)	(3,750)	(3,750)	(3,750)	(3,750)	(3,750)	(15,000)	(15,000)	(13,750)
Net Income Available to Common Stock and Participating Securities	$ 69,721	$ 75,760	$ 74,321	$ 78,424	$ 75,958	$ 75,132	$ 74,976	$ 72,438	$ 298,226	$ 298,504	$ 285,012
Earnings per Common Share - Basic and Diluted (in dollars per share)	$ 0.19	$ 0.20	$ 0.20	$ 0.21	$ 0.20	$ 0.20	$ 0.20	$ 0.20	$ 0.80	$ 0.81	$ 0.78