Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) - USD ($) $ / shares in Units, $ in Thousands	6 Months Ended	12 Months Ended
	Jun. 30, 2015	Dec. 31, 2014
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 7,542,090	$ 6,837,571
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	7,455	4,177
Financial derivatives, at fair value	69,373	84,844
Derivative, Collateral [Abstract]
Collateral Pledged	35,335	46,627
Derivative Asset	7,455	4,177
Derivative Liability	34,038	38,217
Cash [Member]
Derivative, Collateral [Abstract]
Obligation to Return Securities Received as Collateral	0	0
Collateral Pledged	35,300	46,600
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	1,026,075	1,000,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	963	0
Financial derivatives, at fair value	$ 24,366	$ 31,718
Derivative, Weighted-Average Pay Rate	2.46%	2.47%
Derivative, Weighted-Average Receive Rate	0.28%	0.23%
Derivative, Average Remaining Maturity	3 years 7 months 24 days	3 years 11 months 23 days
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 92,000	$ 15,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	866	0
Financial derivatives, at fair value	$ 335	$ 289
Derivative, Weighted-Average Pay Rate	2.16%	2.43%
Derivative, Weighted-Average Receive Rate	0.48%	0.51%
Derivative, Average Remaining Maturity	6 years 11 months 5 days	6 years 2 months 23 days
Not Designated as Hedging Instrument [Member]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Credit Risk Valuation Adjustment, Derivative Assets	$ 20	$ 22
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	247	187
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	519,408	490,183
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	1,425	537
Financial derivatives, at fair value	$ 43,645	$ 51,224
Derivative, Weighted-Average Pay Rate	4.06%	4.23%
Derivative, Weighted-Average Receive Rate	0.28%	0.23%
Derivative, Average Remaining Maturity	8 years 9 months 18 days	7 years 18 days
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 4,062,734	$ 3,829,355
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	4,093	3,414
Financial derivatives, at fair value	$ 313	$ 461
Derivative, Weighted-Average Pay Rate	0.16%	0.14%
Derivative, Weighted-Average Receive Rate	0.35%	0.27%
Derivative, Average Remaining Maturity	6 months 29 days	6 months 18 days
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 487,565	$ 383,565
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	54	1
Financial derivatives, at fair value	$ 437	$ 877
Derivative, Weighted-Average Pay Rate	0.13%	0.12%
Derivative, Weighted-Average Receive Rate	0.82%	1.34%
Derivative, Average Remaining Maturity	2 years 6 months 4 days	3 years 5 months 19 days
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 1,295,000	$ 1,105,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	74	247
Financial derivatives, at fair value	$ 258	$ 406
Derivative, Weighted-Average Pay Rate	0.12%	0.11%
Derivative, Weighted-Average Receive Rate	0.29%	0.31%
Derivative, Average Remaining Maturity	2 years 8 months 5 days	2 years 5 months 1 day
Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 45,608	$ 12,768
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	$ 208	$ 53
Derivative, Weighted-Average Forward Price	$ 99.05	$ 101.00
Not Designated as Hedging Instrument [Member] | Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 13,700	$ 1,700
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	$ 58	$ 3
Derivative, Weighted-Average Forward Price	$ 125.75	$ 126.60