Financial Derivatives - Summary of Derivatives (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Notional Amount	$ 6,107,033	$ 6,586,530
Financial derivatives, at fair value	36,190	40,250
Financial derivatives, at fair value	(164,949)	(160,024)
Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Credit valuation adjustment, Asset	(4)	(773)
Credit valuation adjustment, Liability	388	935
Interest Rate Swap, Pay Fixed Non-Callable [Member] | Fair Value Hedging [Member]
Derivative [Line Items]
Weighted-Average Pay Rate	2.20%
Weighted-Average Receive Rate	0.44%
Derivative, Average Remaining Maturity	4 years 3 months 29 days
Interest Rate Swap, Pay Fixed Non-Callable [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	807,606	1,906,123
Financial derivatives, at fair value	47	0
Financial derivatives, at fair value	(100,814)	(157,520)
Weighted-Average Pay Rate	4.85%	3.65%
Weighted-Average Receive Rate	0.41%	0.46%
Derivative, Average Remaining Maturity	4 years 3 months 22 days	4 years 5 months 23 days
Interest Rate Swap, Receive Fixed Non-Callable | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	3,668,717	4,212,713
Financial derivatives, at fair value	35,632	41,006
Financial derivatives, at fair value	0	(1,302)
Weighted-Average Pay Rate	0.36%	0.41%
Weighted-Average Receive Rate	0.95%	0.96%
Derivative, Average Remaining Maturity	9 months 7 days	11 months 19 days
Interest Rate Swap, Receive Fixed Callable [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	90,000
Financial derivatives, at fair value	7
Financial derivatives, at fair value	(21)
Weighted-Average Pay Rate	0.17%
Weighted-Average Receive Rate	0.60%
Derivative, Average Remaining Maturity	3 years 11 months 26 days
Basis Swaps | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	564,262	457,694
Financial derivatives, at fair value	508	0
Financial derivatives, at fair value	(945)	(2,137)
Weighted-Average Pay Rate	0.53%	0.80%
Weighted-Average Receive Rate	0.38%	0.38%
Derivative, Average Remaining Maturity	1 year 11 months 26 days	1 year 3 months 18 days
Agency Forward [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	20,048
Financial derivatives, at fair value	0
Financial derivatives, at fair value	58
Derivative, Weighted-Average Forward Price	$ 101.17
Treasury Futures | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount	6,400
Financial derivatives, at fair value	0
Financial derivatives, at fair value	3
Derivative, Weighted-Average Forward Price	$ 133.44
Credit Default Swaps | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional Amount		10,000
Financial derivatives, at fair value		17
Financial derivatives, at fair value		$ 0
Weighted-Average Pay Rate		1.00%
Weighted-Average Receive Rate		0.00%
Derivative, Average Remaining Maturity		8 months 19 days