Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) - USD ($) $ / shares in Units, $ in Thousands	9 Months Ended	12 Months Ended
	Sep. 30, 2015	Dec. 31, 2014
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 8,052,133	$ 6,837,571
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	7,027	4,177
Financial derivatives, at fair value	94,880	84,844
Derivative, Collateral [Abstract]
Collateral Pledged	48,111	46,627
Derivative Asset	7,027	4,177
Derivative Liability	46,769	38,217
Cash [Member]
Derivative, Collateral [Abstract]
Obligation to Return Securities Received as Collateral	0	0
Collateral Pledged	48,100	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	1,226,075	1,000,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	$ 36,048	$ 31,718
Derivative, Weighted-Average Pay Rate	2.24%	2.47%
Derivative, Weighted-Average Receive Rate	0.30%	0.23%
Derivative, Average Remaining Maturity	3 years 3 months 22 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	$ 102,000	$ 15,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	$ 2,720	$ 289
Derivative, Weighted-Average Pay Rate	2.23%	2.43%
Derivative, Weighted-Average Receive Rate	0.50%	0.51%
Derivative, Average Remaining Maturity	6 years 11 months 26 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	$ 1	$ 22
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	302	187
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	514,969	490,183
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	16	537
Financial derivatives, at fair value	$ 55,941	$ 51,224
Derivative, Weighted-Average Pay Rate	4.02%	4.23%
Derivative, Weighted-Average Receive Rate	0.30%	0.23%
Derivative, Average Remaining Maturity	8 years 8 months 8 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	$ 5,081,396	$ 3,829,355
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	6,652	3,414
Financial derivatives, at fair value	$ 46	$ 461
Derivative, Weighted-Average Pay Rate	0.19%	0.14%
Derivative, Weighted-Average Receive Rate	0.40%	0.27%
Derivative, Average Remaining Maturity	7 months 2 days	6 months 18 days
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 287,565	$ 383,565
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	108	1
Financial derivatives, at fair value	$ 3	$ 877
Derivative, Weighted-Average Pay Rate	0.17%	0.12%
Derivative, Weighted-Average Receive Rate	0.88%	1.34%
Derivative, Average Remaining Maturity	2 years 9 months 4 days	3 years 5 months 19 days
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 800,000	$ 1,105,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	252	247
Financial derivatives, at fair value	$ 60	$ 406
Derivative, Weighted-Average Pay Rate	0.15%	0.11%
Derivative, Weighted-Average Receive Rate	0.28%	0.31%
Derivative, Average Remaining Maturity	2 years 6 months 11 days	2 years 5 months 1 day
Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 28,528	$ 12,768
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	$ 312	$ 53
Derivative, Weighted-Average Forward Price	$ 100.50	$ 101.00
Not Designated as Hedging Instrument [Member] | Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 11,600	$ 1,700
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	$ 52	$ 3
Derivative, Weighted-Average Forward Price	$ 128.28	$ 126.60