Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended	12 Months Ended
	Mar. 31, 2016	Dec. 31, 2015
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 8,949,663	$ 8,429,964
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	7,034	3,816
Financial derivatives, at fair value	117,956	77,199
Derivative, Collateral [Abstract]
Collateral Pledged	73,659	37,986
Derivative Asset	7,034	3,816
Derivative Liability	44,297	39,213
Cash [Member]
Derivative, Collateral [Abstract]
Obligation to Return Securities Received as Collateral	0	0
Collateral Pledged	73,700	38,000
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	1,391,816	1,276,285
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	207	949
Financial derivatives, at fair value	$ 52,859	$ 26,703
Derivative, Average Fixed Interest Rate	1.83%	2.35%
Derivative, Average Variable Interest Rate	0.62%	0.37%
Derivative, Average Remaining Maturity	5 years 5 months 26 days	4 years 1 month 28 days
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 144,000	$ 119,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	8
Financial derivatives, at fair value	$ 6,282	$ 1,381
Derivative, Average Fixed Interest Rate	2.25%	2.25%
Derivative, Average Variable Interest Rate	0.81%	0.64%
Derivative, Average Remaining Maturity	7 years 18 days	7 years 11 days
Not Designated as Hedging Instrument [Member]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Credit Risk Valuation Adjustment, Derivative Assets	$ 0	$ 5
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	303	170
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	445,041	454,041
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	229
Financial derivatives, at fair value	$ 57,430	$ 44,528
Derivative, Average Fixed Interest Rate	4.10%	3.73%
Derivative, Average Variable Interest Rate	0.62%	0.33%
Derivative, Average Remaining Maturity	6 years 6 months 11 days	6 years 7 days
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 6,156,403	$ 5,590,638
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	6,629	2,384
Financial derivatives, at fair value	$ 1,332	$ 4,205
Derivative, Average Fixed Interest Rate	0.56%	0.47%
Derivative, Average Variable Interest Rate	0.50%	0.31%
Derivative, Average Remaining Maturity	7 months 10 days	6 months 25 days
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 115,000	$ 230,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	135	0
Financial derivatives, at fair value	$ 47	$ 421
Derivative, Average Fixed Interest Rate	1.30%	0.91%
Derivative, Average Variable Interest Rate	0.56%	0.41%
Derivative, Average Remaining Maturity	3 years	2 years 3 months 4 days
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 675,000	$ 725,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	63	232
Financial derivatives, at fair value	$ 130	$ 131
Derivative, Average Variable Interest Rate	0.48%	0.22%
Derivative, Variable Interest Rate	0.57%	0.38%
Derivative, Average Remaining Maturity	1 year 11 months 5 days	2 years 3 months 29 days
Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 16,603
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0
Financial derivatives, at fair value	$ 134
Derivative, Weighted-Average Forward Price	$ 102.22
Not Designated as Hedging Instrument [Member] | Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 5,800	$ 35,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	19
Financial derivatives, at fair value	$ 45	$ 0
Derivative, Weighted-Average Forward Price	$ 129.62	$ 125.96