Financial Derivatives (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	$ 6,731,092	$ 6,815,268
Financial derivatives, at fair value	23,164	31,173
Financial derivatives, at fair value	99,063	150,682
Derivative Assets	21,806	29,523
Derivative Liabilities	(78,784)	(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	900,000	950,000
Fair Value Hedge Assets	0	0
Fair Value Hedge Liabilities	35,202	58,758
Derivative, Weighted-Average Pay Rate	2.25%
Derivative, Weighted-Average Receive Rate	0.27%
Derivative, Average Remaining Maturity	3 years 9 months 0 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	157	27
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	(432)	(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	4.62%	4.83%
Derivative, Weighted-Average Receive Rate	0.27%	0.32%
Derivative, Average Remaining Maturity	3 years 8 months 23 days	4 years 1 month 20 days
Derivative, Notional Amount	799,926	805,622
Financial derivatives, at fair value	2,951	357
Financial derivatives, at fair value	61,043	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	0.80%	0.85%
Derivative, Average Remaining Maturity	0 years 8 months 16 days	8 months 26 days
Derivative, Notional Amount	4,283,440	4,135,149
Financial derivatives, at fair value	19,910	30,338
Financial derivatives, at fair value	834	211
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Callable [Member]
Derivatives, Fair Value [Line Items]
Derivative, Weighted-Average Pay Rate	0.13%	0.15%
Derivative, Weighted-Average Receive Rate	0.52%	0.55%
Derivative, Average Remaining Maturity	3 years 3 months 0 days	3 years 10 months 20 days
Derivative, Notional Amount	245,000	245,000
Financial derivatives, at fair value	0	6
Financial derivatives, at fair value	1,933	238
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Weighted-Average Pay Rate	0.42%	0.43%
Derivative, Weighted-Average Receive Rate	0.30%	0.36%
Derivative, Average Remaining Maturity	1 year 4 months 13 days	1 year 3 months 14 days
Derivative, Notional Amount	450,223	609,262
Financial derivatives, at fair value	328	499
Financial derivatives, at fair value	453	784
Not Designated as Hedging Instrument [Member] | Agency Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	43,603	59,035
Financial derivatives, at fair value	132	0
Financial derivatives, at fair value	0	58
Derivative, Weighted-Average Forward Price	$ 97.38	$ 101.22
Not Designated as Hedging Instrument [Member] | Treasury Futures [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	8,900	11,200
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	30	12
Derivative, Weighted-Average Forward Price	$ 126.23	$ 129.77
Cash [Member]
Derivatives, Fair Value [Line Items]
Obligation to Return Securities Received as Collateral	1,358	1,650
Collateral Already Posted, Aggregate Fair Value	$ 20,279	$ 60,311