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, 2012	Sep. 30, 2012	Jun. 30, 2012	Mar. 31, 2012	Dec. 31, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Summary of Quarterly Results of Operations (Unaudited)
Interest Income	$ 124,968	$ 125,135	$ 125,531	$ 123,523	$ 123,994	$ 130,766	$ 132,109	$ 109,878	$ 499,157	$ 496,747	$ 391,338
Interest Expense	(43,054)	(45,801)	(42,688)	(40,127)	(38,811)	(38,752)	(37,195)	(34,653)	(171,670)	(149,411)	(145,125)
Net Interest Income	81,914	79,334	82,843	83,396	85,183	92,014	94,914	75,225	327,487	347,336	246,213
Net Impairment Losses Recognized in Earnings			(280)	(920)	(4,187)	(3,991)	(2,392)		(1,200)	(10,570)	(12,277)
Gain on Sale of MBS	1,769	4,279		2,953	2,534	4,196			9,001	6,730	33,739
Unrealized net gains and net interest income from Linked Transactions	1,166	3,177	568	7,699	(6,955)	733	(5,613)	14,850	12,610	3,015	53,762
Other Income	8	1	1		822	(508)	387	381
Operating and Other Expense	(15,638)	(8,650)	(8,366)	(8,415)	(6,833)	(8,745)	(8,010)	(7,591)	(41,069)	(31,179)	(26,324)
Net Income	69,219	78,141	74,766	84,713	70,564	83,699	79,286	82,865	306,839	316,414	269,762
Preferred Stock Dividends	(2,040)	(2,040)	(2,040)	(2,040)	(2,040)	(2,040)	(2,040)	(2,040)	(8,160)	(8,160)	(8,160)
Net Income Available to Common Stock and Participating Securities	$ 67,179	$ 76,101	$ 72,726	$ 82,673	$ 68,524	$ 81,659	$ 77,246	$ 80,825	$ 298,679	$ 308,254	$ 261,602
Earnings Per Common Share - Basic and Diluted	$ 0.19	$ 0.21	$ 0.20	$ 0.23	$ 0.19	$ 0.23	$ 0.22	$ 0.27	$ 0.83	$ 0.90	$ 0.93