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, 2016	Dec. 31, 2015
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 8,532,790	$ 8,429,964
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	8,242	3,816
Financial derivatives, at fair value	(140,758)	(77,199)
Derivative, Collateral [Abstract]
Collateral Pledged	95,144	37,986
Derivative Asset	8,242	3,816
Derivative Liability	45,614	39,213
Cash [Member]
Derivative, Collateral [Abstract]
Obligation to Return Securities Received as Collateral	0	0
Collateral Pledged	95,100	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	6	949
Financial derivatives, at fair value	$ (67,960)	$ (26,703)
Derivative, Average Fixed Interest Rate	1.83%	2.35%
Derivative, Average Variable Interest Rate	0.64%	0.37%
Derivative, Average Remaining Maturity	5 years 2 months 26 days	4 years 1 month 28 days
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member] | Interest Rate Swap, Receive Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 30,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	862
Financial derivatives, at fair value	$ 0
Derivative, Average Fixed Interest Rate	0.78%
Derivative, Average Variable Interest Rate	1.75%
Derivative, Average Remaining Maturity	3 years 11 months 16 days
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 157,000	$ 119,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	8
Financial derivatives, at fair value	$ (9,360)	$ (1,381)
Derivative, Average Fixed Interest Rate	2.26%	2.25%
Derivative, Average Variable Interest Rate	0.86%	0.64%
Derivative, Average Remaining Maturity	7 years 2 months 16 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	165	170
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	442,611	454,041
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	229
Financial derivatives, at fair value	$ (61,957)	$ (44,528)
Derivative, Average Fixed Interest Rate	4.08%	3.73%
Derivative, Average Variable Interest Rate	0.64%	0.33%
Derivative, Average Remaining Maturity	6 years 3 months 22 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	$ 6,035,677	$ 5,590,638
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	7,367	2,384
Financial derivatives, at fair value	$ (1,007)	$ (4,205)
Derivative, Average Fixed Interest Rate	0.60%	0.47%
Derivative, Average Variable Interest Rate	0.51%	0.31%
Derivative, Average Remaining Maturity	6 months 11 days	6 months 25 days
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 30,000	$ 230,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	$ (17)	$ (421)
Derivative, Average Fixed Interest Rate	0.58%	0.91%
Derivative, Average Variable Interest Rate	0.57%	0.41%
Derivative, Average Remaining Maturity	9 months 29 days	2 years 3 months 4 days
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 400,000	$ 725,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	7	232
Financial derivatives, at fair value	$ (219)	$ (131)
Derivative, Average Variable Interest Rate	0.61%	0.22%
Derivative, Variable Interest Rate	0.58%	0.38%
Derivative, Average Remaining Maturity	11 months 12 days	2 years 3 months 29 days
Not Designated as Hedging Instrument [Member] | Forward Contracts [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 17,586
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0
Financial derivatives, at fair value	$ (94)
Derivative, Weighted-Average Forward Price	$ 101.03
Not Designated as Hedging Instrument [Member] | Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 28,100	$ 35,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	19
Financial derivatives, at fair value	$ (309)	$ 0
Derivative, Weighted-Average Forward Price	$ 131.89	$ 125.96