Derivatives (Details 2) (Linked Transactions, USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Linked Repurchase Agreements
Balance	$ 170,897	$ 567,287
Weighted Average Interest Rate (as a percent)	1.88%	1.62%
Linked MBS
Fair Value	225,969	744,369
Amortized Cost	237,595	718,734
Par/Current Face	289,536	863,280
Weighted Average Coupon Rate (as a percent)	4.74%	4.56%
Components of the gain on Linked Transactions
Interest income attributable to MBS underlying Linked Transactions	25,592	35,313	6,249
Interest expense attributable to linked repurchase agreement borrowings underlying Linked Transactions	(4,794)	(6,432)	(1,254)
Change in fair value of Linked Transactions included in earnings	(17,783)	24,881	3,834
Unrealized net (losses)/gains and net interest income from Linked Transactions	3,015	53,762	8,829
Rated AAA
Linked MBS
Fair Value	29,057	46,710
Amortized Cost	29,917	46,367
Par/Current Face	30,675	47,151
Weighted Average Coupon Rate (as a percent)	3.31%	4.13%
Rated AA
Linked MBS
Fair Value	17,427	57,634
Amortized Cost	16,858	54,176
Par/Current Face	17,297	61,389
Weighted Average Coupon Rate (as a percent)	5.00%	3.51%
Rated A
Linked MBS
Fair Value		36,440
Amortized Cost		34,620
Par/Current Face		41,984
Weighted Average Coupon Rate (as a percent)		2.53%
Rated BBB
Linked MBS
Fair Value	41,825	69,397
Amortized Cost	42,419	66,848
Par/Current Face	49,781	78,741
Weighted Average Coupon Rate (as a percent)	2.81%	3.38%
Rated BB
Linked MBS
Fair Value		14,536
Amortized Cost		14,456
Par/Current Face		17,513
Weighted Average Coupon Rate (as a percent)		2.51%
Rated B
Linked MBS
Fair Value		129,962
Amortized Cost		121,198
Par/Current Face		139,763
Weighted Average Coupon Rate (as a percent)		4.28%
Rated CCC
Linked MBS
Fair Value	20,782	216,398
Amortized Cost	20,988	211,302
Par/Current Face	26,680	255,667
Weighted Average Coupon Rate (as a percent)	4.42%	4.98%
Rated CC
Linked MBS
Fair Value	43,644	89,833
Amortized Cost	47,060	86,509
Par/Current Face	61,470	110,518
Weighted Average Coupon Rate (as a percent)	6.00%	5.45%
Rated C
Linked MBS
Fair Value	32,870	78,181
Amortized Cost	36,934	78,038
Par/Current Face	45,857	100,204
Weighted Average Coupon Rate (as a percent)	5.20%	5.77%
D-rated
Linked MBS
Fair Value	40,364
Amortized Cost	43,419
Par/Current Face	57,776
Weighted Average Coupon Rate (as a percent)	5.54%
Not Rated
Linked MBS
Fair Value		5,278
Amortized Cost		5,220
Par/Current Face		10,350
Weighted Average Coupon Rate (as a percent)		6.00%
Within 30 days
Linked Repurchase Agreements
Balance	141,719	289,522
Weighted Average Interest Rate (as a percent)	1.89%	1.62%
Over 30 days to 90 days
Linked Repurchase Agreements
Balance	$ 29,178	$ 277,765
Weighted Average Interest Rate (as a percent)	1.81%	1.62%