Financial Derivatives - Summary of Derivatives (Details) (USD $) In Thousands, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2012	Dec. 31, 2011
Derivative [Line Items]
Derivative Asset, Fair Value, Gross Asset	$ 36,894	$ 40,250
Derivative Liability, Fair Value, Gross Liability	162,591	160,024
Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	6,310,269	6,586,530
Derivative Asset, Fair Value, Gross Asset	36,894	40,250
Derivative Liability, Fair Value, Gross Liability	162,591	160,024
Credit valuation adjustment, Asset	0	(773)
Credit valuation adjustment, Liability	673	935
Interest Rate Swap, Pay Fixed Non-Callable | Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	1,789,433	1,906,123
Derivative Asset, Fair Value, Gross Asset	4	0
Derivative Liability, Fair Value, Gross Liability	162,126	157,520
Weighted-Average Pay Rate	3.47%	3.65%
Weighted-Average Receive Rate	0.47%	0.46%
Derivative, Average Remaining Maturity	4 years 4 months 24 days	4 years 5 months 22 days
Interest Rate Swap, Receive Fixed Non-Callable | Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	3,915,717	4,212,713
Derivative Asset, Fair Value, Gross Asset	36,732	41,006
Derivative Liability, Fair Value, Gross Liability	57	1,302
Weighted-Average Pay Rate	0.36%	0.41%
Weighted-Average Receive Rate	0.90%	0.96%
Derivative, Average Remaining Maturity	10 months 2 days	11 months 19 days
Basis Swaps | Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	595,119	457,694
Derivative Asset, Fair Value, Gross Asset	158	0
Derivative Liability, Fair Value, Gross Liability	1,073	2,137
Weighted-Average Pay Rate	0.60%	0.80%
Weighted-Average Receive Rate	0.38%	0.38%
Derivative, Average Remaining Maturity	1 year 3 months 14 days	1 year 3 months 18 days
Credit Default Swaps | Not Designated as Hedging Instrument
Derivative [Line Items]
Notional Amount	10,000	10,000
Derivative Asset, Fair Value, Gross Asset	0	17
Derivative Liability, Fair Value, Gross Liability	$ 8	$ 0
Weighted-Average Pay Rate	1.00%	1.00%
Weighted-Average Receive Rate	0.00%	0.00%
Derivative, Average Remaining Maturity	2 months 19 days	8 months 19 days