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, 2017	Dec. 31, 2016
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 8,791,110	$ 8,100,257
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	2,020	23,182
Financial derivatives, at fair value	(30,595)	(58,152)
Derivative, Collateral [Abstract]
Collateral Pledged	25,079	25,643
Derivative Asset	2,020	23,182
Derivative Liability	5,516	32,509
Cash [Member]
Derivative, Collateral [Abstract]
Obligation to Return Securities Received as Collateral	0	0
Collateral Pledged	100	1,000
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	2,040,854	1,642,609
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	744	18,508
Financial derivatives, at fair value	$ (8,768)	$ (18,909)
Derivative, Average Fixed Interest Rate	1.83%	1.73%
Derivative, Average Variable Interest Rate	1.31%	0.90%
Derivative, Average Remaining Maturity	5 years 5 months 2 days	4 years 8 months 13 days
Designated as Hedging Instrument [Member] | Fair Value Hedging [Member] | Interest Rate Swap, Receive Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 1,431,700
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	64
Financial derivatives, at fair value	$ (1,985)
Derivative, Average Fixed Interest Rate	1.23%
Derivative, Average Variable Interest Rate	1.42%
Derivative, Average Remaining Maturity	1 year 8 months 25 days
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 349,000	$ 207,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	960	3,706
Financial derivatives, at fair value	$ (555)	$ (955)
Derivative, Average Fixed Interest Rate	2.19%	2.18%
Derivative, Average Variable Interest Rate	1.55%	1.11%
Derivative, Average Remaining Maturity	6 years 3 months 25 days	7 years 3 months 10 days
Not Designated as Hedging Instrument [Member]
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Derivative Credit Risk Valuation Adjustment, Derivative Assets	$ 6	$ 14
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	94	158
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	348,040	435,827
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	166	339
Financial derivatives, at fair value	$ (19,318)	$ (32,951)
Derivative, Average Fixed Interest Rate	3.80%	4.06%
Derivative, Average Variable Interest Rate	1.31%	0.89%
Derivative, Average Remaining Maturity	6 years 10 months 16 days	5 years 10 months 24 days
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 3,389,116	$ 4,991,821
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	607
Financial derivatives, at fair value	$ 0	$ (5,064)
Derivative, Average Fixed Interest Rate	1.10%	0.75%
Derivative, Average Variable Interest Rate	1.12%	0.74%
Derivative, Average Remaining Maturity	10 months 5 days	7 months 5 days
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Receive Fixed 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		0
Financial derivatives, at fair value		$ (33)
Derivative, Average Fixed Interest Rate		0.58%
Derivative, Average Variable Interest Rate		0.82%
Derivative, Average Remaining Maturity		4 months
Not Designated as Hedging Instrument [Member] | Basis Swap [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 1,225,000	$ 765,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	42	36
Financial derivatives, at fair value	$ (63)	$ (243)
Derivative, Average Variable Interest Rate	1.20%	0.78%
Derivative, Variable Interest Rate	1.22%	0.78%
Derivative, Average Remaining Maturity	11 months 11 days	10 months 15 days
Not Designated as Hedging Instrument [Member] | Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 7,400	$ 28,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	50	0
Financial derivatives, at fair value	$ 0	$ (155)
Derivative, Weighted-Average Forward Price	$ 125.99	$ 123.73