Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	3 Months Ended	12 Months Ended
	Mar. 31, 2014	Dec. 31, 2013
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	$ 6,628,072	$ 6,691,851
Financial derivatives Assets, at fair value	11,357	19,718
Financial derivatives Liability, at fair value	73,887	75,708
Collateral Pledged	14,223	11,320
Derivative Asset	11,357	19,718
Financial derivatives, Liabilities	59,664	64,388
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Credit Risk Valuation Adjustment, Derivative Assets	117	201
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	222	156
Interest Rate Swap, Pay Fixed Non-Callable [Member] | Fair Value Hedging [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	900,000	900,000
Fair Value Hedge Assets	0	0
Fair Value Hedge Liabilities	28,789	28,989
Derivative, Weighted-Average Pay Rate	2.25%	2.25%
Derivative, Weighted-Average Receive Rate	0.24%	0.24%
Derivative, Average Remaining Maturity	3 years 0 months 0 days	3 years 3 months 0 days
Interest Rate Swap, Pay Fixed Non-Callable [Member] | Cash Flow Hedging [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	10,000	10,000
Cash Flow Hedge Assets at Fair Value	0	68
Cash Flow Hedge Liabilities at Fair Value	(36)	0
Derivative, Weighted-Average Pay Rate	2.50%	2.50%
Derivative, Weighted-Average Receive Rate	0.48%	0.48%
Derivative, Average Remaining Maturity	6 years 8 months 12 days	6 years 11 months 12 days
Interest Rate Swap, Pay Fixed Non-Callable [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	507,820	806,596
Derivative, Weighted-Average Pay Rate	4.28%	4.63%
Derivative, Weighted-Average Receive Rate	0.24%	0.24%
Derivative, Average Remaining Maturity	7 years 6 months 18 days	4 years 10 months 10 days
Financial derivatives Assets, at fair value	3,074	7,570
Financial derivatives Liability, at fair value	43,448	45,360
Interest Rate Swap, Receive Fixed Non-Callable [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	4,431,663	4,324,663
Derivative, Weighted-Average Pay Rate	0.26%	0.27%
Derivative, Weighted-Average Receive Rate	0.68%	0.70%
Derivative, Average Remaining Maturity	0 years 5 months 19 days	0 years 6 months 11 days
Financial derivatives Assets, at fair value	8,045	11,836
Financial derivatives Liability, at fair value	183	262
Interest Rate Swap, Receive Fixed Callable [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	195,000	175,000
Derivative, Weighted-Average Pay Rate	0.10%	0.10%
Derivative, Weighted-Average Receive Rate	0.65%	0.65%
Derivative, Average Remaining Maturity	3 years 0 months 11 days	3 years 3 months 18 days
Financial derivatives Assets, at fair value	4	83
Financial derivatives Liability, at fair value	1,081	934
Basis Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	565,189	404,288
Derivative, Weighted-Average Pay Rate	0.25%	0.32%
Derivative, Weighted-Average Receive Rate	0.26%	0.29%
Derivative, Average Remaining Maturity	1 year 10 months 10 days	1 year 6 months 7 days
Financial derivatives Assets, at fair value	336	276
Financial derivatives Liability, at fair value	572	318
Agency Forward [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	0	65,704
Financial derivatives Assets, at fair value	0	86
Financial derivatives Liability, at fair value	0	0
Derivative, Weighted-Average Forward Price	$ 0.00	$ 98.91
Treasury Futures [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount	18,400	5,600
Financial derivatives Assets, at fair value	15	0
Financial derivatives Liability, at fair value	0	1
Derivative, Weighted-Average Forward Price	$ 123.58	$ 123.02
Cash [Member]
Derivatives, Fair Value [Line Items]
Collateral Received	0	0
Collateral Pledged	$ 14,200	$ 9,800