Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) (USD $) In Thousands, except Per Share data, unless otherwise specified	6 Months Ended	12 Months Ended
	Jun. 30, 2013	Dec. 31, 2013	Jun. 30, 2014
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount		$ 6,691,851	$ 6,495,920
Financial derivatives Assets, at fair value		19,718	7,318
Financial derivatives Liability, at fair value		75,708	78,296
Collateral Pledged		11,320	23,822
Derivative Asset		19,718	7,318
Financial derivatives, Liabilities		64,388	54,474
Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative Credit Risk Valuation Adjustment, Derivative Assets		201	56
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities		156	129
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		28,989	29,589
Derivative, Weighted-Average Pay Rate		2.25%	2.25%
Derivative, Weighted-Average Receive Rate		0.24%	0.23%
Derivative, Average Remaining Maturity	2 years 9 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		68	0
Cash Flow Hedge Liabilities at Fair Value		0	(129)
Derivative, Weighted-Average Pay Rate		2.50%	2.50%
Derivative, Weighted-Average Receive Rate		0.48%	0.50%
Derivative, Average Remaining Maturity	6 years 5 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		806,596	507,552
Derivative, Weighted-Average Pay Rate		4.63%	4.27%
Derivative, Weighted-Average Receive Rate		0.24%	0.23%
Derivative, Average Remaining Maturity	7 years 4 months 2 days	4 years 10 months 10 days
Financial derivatives Assets, at fair value		7,570	1,955
Financial derivatives Liability, at fair value		45,360	47,646
Interest Rate Swap, Receive Fixed Non-Callable [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount		4,324,663	3,968,832
Derivative, Weighted-Average Pay Rate		0.27%	0.19%
Derivative, Weighted-Average Receive Rate		0.70%	0.39%
Derivative, Average Remaining Maturity	0 years 5 months 23 days	0 years 6 months 11 days
Financial derivatives Assets, at fair value		11,836	4,979
Financial derivatives Liability, at fair value		262	186
Interest Rate Swap, Receive Fixed Callable [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount		175,000	200,000
Derivative, Weighted-Average Pay Rate		0.10%	0.09%
Derivative, Weighted-Average Receive Rate		0.65%	0.98%
Derivative, Average Remaining Maturity	3 years 5 months 26 days	3 years 3 months 18 days
Financial derivatives Assets, at fair value		83	22
Financial derivatives Liability, at fair value		934	424
Basis Swap [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount		404,288	880,000
Derivative, Weighted-Average Pay Rate		0.32%	0.12%
Derivative, Weighted-Average Receive Rate		0.29%	0.30%
Derivative, Average Remaining Maturity	2 years 3 months 14 days	1 year 6 months 7 days
Financial derivatives Assets, at fair value		276	418
Financial derivatives Liability, at fair value		318	240
Agency Forward [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount		65,704	15,836
Financial derivatives Assets, at fair value		86	0
Financial derivatives Liability, at fair value		0	149
Derivative, Weighted-Average Forward Price	$ 98.27	$ 98.91
Treasury Futures [Member] | Not Designated as Hedging Instrument [Member]
Derivatives, Fair Value [Line Items]
Derivative, Notional Amount		5,600	13,700
Financial derivatives Assets, at fair value		0	0
Financial derivatives Liability, at fair value		1	62
Derivative, Weighted-Average Forward Price	$ 124.72	$ 123.02
Cash [Member]
Derivatives, Fair Value [Line Items]
Collateral Received		0	0
Collateral Pledged		$ 9,800	$ 23,800