Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 8,100,257	$ 8,429,964
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	23,182	3,816
Financial derivatives, at fair value	(58,152)	(77,199)
Derivative Credit Risk Valuation Adjustment, Derivative Assets	100	200
Derivative, Collateral [Abstract]
Collateral Pledged	25,643	37,986
Derivative Asset	23,182	3,816
Derivative Liability	32,509	39,213
Cash [Member]
Derivative, Collateral [Abstract]
Obligation to Return Securities Received as Collateral	0	0
Collateral Pledged	1,000	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	1,642,609	1,276,285
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	18,508	949
Financial derivatives, at fair value	$ (18,909)	$ (26,703)
Derivative, Average Fixed Interest Rate	1.73%	2.35%
Derivative, Average Variable Interest Rate	0.90%	0.37%
Derivative, Average Remaining Maturity	4 years 8 months 13 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	$ 207,000	$ 119,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	3,706	8
Financial derivatives, at fair value	$ (955)	$ (1,381)
Derivative, Average Fixed Interest Rate	2.18%	2.25%
Derivative, Average Variable Interest Rate	1.11%	0.64%
Derivative, Average Remaining Maturity	7 years 3 months 10 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	$ 14	$ 5
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	158	170
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	435,827	454,041
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	339	229
Financial derivatives, at fair value	$ (32,951)	$ (44,528)
Derivative, Average Fixed Interest Rate	4.06%	3.73%
Derivative, Average Variable Interest Rate	0.89%	0.33%
Derivative, Average Remaining Maturity	5 years 10 months 24 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	$ 4,991,821	$ 5,590,638
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	607	2,384
Financial derivatives, at fair value	$ (5,064)	$ (4,205)
Derivative, Average Fixed Interest Rate	0.75%	0.47%
Derivative, Average Variable Interest Rate	0.74%	0.31%
Derivative, Average Remaining Maturity	7 months 5 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	$ (33)	$ (421)
Derivative, Average Fixed Interest Rate	0.58%	0.91%
Derivative, Average Variable Interest Rate	0.82%	0.41%
Derivative, Average Remaining Maturity	4 months	2 years 3 months 4 days
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 765,000	$ 725,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	36	232
Financial derivatives, at fair value	$ (243)	$ (131)
Derivative, Average Variable Interest Rate	0.78%	0.22%
Derivative, Variable Interest Rate	0.78%	0.38%
Derivative, Average Remaining Maturity	10 months 15 days	2 years 3 months 29 days
Not Designated as Hedging Instrument [Member] | Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 28,000	$ 35,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	19
Financial derivatives, at fair value	$ (155)	$ 0
Derivative, Weighted-Average Forward Price	$ 123.73	$ 125.96