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, 2011	Sep. 30, 2011	Jun. 30, 2011	Mar. 31, 2011	Dec. 31, 2010	Sep. 30, 2010	Jun. 30, 2010	Mar. 31, 2010	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Summary of Quarterly Results of Operations (Unaudited)
Interest Income	$ 123,994	$ 130,766	$ 132,109	$ 109,878	$ 97,597	$ 97,417	$ 88,627	$ 107,697	$ 496,747	$ 391,338	$ 505,561
Interest Expense	(38,811)	(38,752)	(37,195)	(34,653)	(35,469)	(35,464)	(35,741)	(38,451)	(149,411)	(145,125)	(229,406)
Net Interest Income	85,183	92,014	94,914	75,225	62,128	61,953	52,886	69,246	347,336	246,213	276,155
Net Impairment Losses Recognized in Earnings	(4,187)	(3,991)	(2,392)		(6,865)		(5,412)		(10,570)	(12,277)	(17,928)
Gain on sale of MBS, net	2,534	4,196						33,739	6,730	33,739	22,617
Unrealized net gains/(losses) and net interest income from Linked Transactions	(6,955)	733	(5,613)	14,850	12,458	21,307	7,197	12,800	3,015	53,762	8,829
Loss on termination of repurchase agreements								(26,815)		(26,815)
Other Income/(Loss)	822	(508)	387	381	364	369	357	374	10,827	62,150	33,009
Operating and Other Expense	(6,833)	(8,745)	(8,010)	(7,591)	(6,504)	(6,415)	(6,738)	(6,667)	(31,179)	(26,324)	(23,047)
Net Income	70,564	83,699	79,286	82,865	61,581	77,214	48,290	82,677	316,414	269,762	268,189
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	$ 68,524	$ 81,659	$ 77,246	$ 80,825	$ 59,541	$ 75,174	$ 46,250	$ 80,637	$ 308,254	$ 261,602	$ 260,029
Earnings Per Common Share - Basic and Diluted	$ 0.19	$ 0.23	$ 0.22	$ 0.27	$ 0.21	$ 0.27	$ 0.16	$ 0.29	$ 0.90	$ 0.93	$ 1.06