Financial Derivatives (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	$ 6,912,555	$ 6,815,268
Financial derivatives, at fair value	26,254	31,173
Financial derivatives, at fair value	133,838	150,682
Derivative Assets	24,759	29,523
Derivative Liabilities	(97,127)	(90,371)
Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Derivatives, Fair Value [Line Items]
Notional Amount of Interest Rate Fair Value Hedge Derivatives	950,000	950,000
Fair Value Hedge Assets	0	0
Fair Value Hedge Liabilities	52,967	58,758
Derivative, Weighted-Average Pay Rate	2.20%
Derivative, Weighted-Average Receive Rate	0.30%
Derivative, Average Remaining Maturity	3 years 9 months 29 days
Fair Value Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Derivatives, Fair Value [Line Items]
Derivative, Weighted-Average Pay Rate		2.20%
Derivative, Weighted-Average Receive Rate		0.31%
Derivative, Average Remaining Maturity		4 years 0 months 25 days
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Credit Risk Valuation Adjustment, Derivative Assets	66	27
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	(744)	(584)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Derivatives, Fair Value [Line Items]
Derivative, Weighted-Average Pay Rate	4.85%	4.83%
Derivative, Weighted-Average Receive Rate	0.29%	0.32%
Derivative, Average Remaining Maturity	4 years 2 months 5 days	4 years 1 month 20 days
Derivative, Notional Amount	749,874	805,622
Financial derivatives, at fair value	857	357
Financial derivatives, at fair value	80,097	91,205
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Non-Callable [Member]
Derivatives, Fair Value [Line Items]
Derivative, Weighted-Average Pay Rate	0.32%	0.33%
Derivative, Weighted-Average Receive Rate	0.81%	0.85%
Derivative, Average Remaining Maturity	8 months 26 days	8 months 26 days
Derivative, Notional Amount	4,355,623	4,135,149
Financial derivatives, at fair value	25,075	30,338
Financial derivatives, at fair value	446	211
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Callable [Member]
Derivatives, Fair Value [Line Items]
Derivative, Weighted-Average Pay Rate	0.14%	0.15%
Derivative, Weighted-Average Receive Rate	0.58%	0.55%
Derivative, Average Remaining Maturity	3 years 9 months 0 days	3 years 10 months 20 days
Derivative, Notional Amount	290,000	245,000
Financial derivatives, at fair value	0	6
Financial derivatives, at fair value	429	238
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Weighted-Average Pay Rate	0.37%	0.43%
Derivative, Weighted-Average Receive Rate	0.31%	0.36%
Derivative, Average Remaining Maturity	1 year 2 months 26 days	1 year 3 months 14 days
Derivative, Notional Amount	550,223	609,262
Financial derivatives, at fair value	375	499
Financial derivatives, at fair value	642	784
Not Designated as Hedging Instrument [Member] | Agency Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	2,435	59,035
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	1	58
Derivative, Weighted-Average Forward Price	$ 99.83	$ 101.22
Not Designated as Hedging Instrument [Member] | Treasury Futures [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	14,400	11,200
Financial derivatives, at fair value	13	0
Financial derivatives, at fair value	0	12
Derivative, Weighted-Average Forward Price	$ 132.08	$ 129.77
Cash [Member]
Derivatives, Fair Value [Line Items]
Obligation to Return Securities Received as Collateral	1,495	1,650
Collateral Already Posted, Aggregate Fair Value	$ (36,711)	$ (60,311)