Financial Derivatives - (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	9 Months Ended	12 Months Ended
	Sep. 30, 2013	Dec. 31, 2012
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	$ 6,592,077	$ 6,815,268
Financial derivatives, at fair value	19,676	31,173
Financial derivatives, at fair value	(91,445)	(150,682)
Collateral Already Posted, Aggregate Fair Value	11,977	60,311
Derivative Assets	18,934	29,523
Derivative Liabilities	(79,468)	(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	35,198	58,758
Derivative, Weighted-Average Pay Rate	2.25%	2.20%
Derivative, Weighted-Average Receive Rate	0.27%	0.31%
Derivative, Average Remaining Maturity	3 years 6 months 0 days	4 years 0 months 25 days
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Credit Risk Valuation Adjustment, Derivative Assets	176	27
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	583	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	4.57%	4.83%
Derivative, Weighted-Average Receive Rate	0.26%	0.32%
Derivative, Average Remaining Maturity	4 years 0 months 11 days	4 years 1 month 20 days
Derivative, Notional Amount	784,349	805,622
Financial derivatives, at fair value	3,398	357
Financial derivatives, at fair value	(55,157)	(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	0.30%	0.33%
Derivative, Weighted-Average Receive Rate	0.78%	0.85%
Derivative, Average Remaining Maturity	0 years 7 months 2 days	8 months 26 days
Derivative, Notional Amount	4,308,440	4,135,149
Financial derivatives, at fair value	16,135	30,338
Financial derivatives, at fair value	(178)	(211)
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Callable [Member]
Derivatives, Fair Value [Line Items]
Derivative, Weighted-Average Pay Rate	0.12%	0.15%
Derivative, Weighted-Average Receive Rate	0.55%	0.55%
Derivative, Average Remaining Maturity	3 years 2 months 19 days	3 years 10 months 20 days
Derivative, Notional Amount	225,000	245,000
Financial derivatives, at fair value	4	6
Financial derivatives, at fair value	(1,137)	(238)
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Derivatives, Fair Value [Line Items]
Derivative, Weighted-Average Pay Rate	0.34%	0.43%
Derivative, Weighted-Average Receive Rate	0.29%	0.36%
Derivative, Average Remaining Maturity	1 year 4 months 2 days	1 year 3 months 14 days
Derivative, Notional Amount	374,288	609,262
Financial derivatives, at fair value	315	499
Financial derivatives, at fair value	(358)	(784)
Not Designated as Hedging Instrument [Member] | Agency Forward [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount		59,035
Financial derivatives, at fair value		0
Financial derivatives, at fair value		(58)
Derivative, Weighted-Average Forward Price		$ 101.22
Not Designated as Hedging Instrument [Member] | Treasury Futures [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount		11,200
Financial derivatives, at fair value		0
Financial derivatives, at fair value		(12)
Derivative, Weighted-Average Forward Price		$ 129.77
Cash [Member]
Derivatives, Fair Value [Line Items]
Obligation to Return Securities Received as Collateral	742	1,650
Collateral Already Posted, Aggregate Fair Value	$ 10,500	$ 60,300