Financial Derivatives - Schedule of Derivative Instruments in Statement of Financial Position (Details) - USD ($) $ / shares in Units, $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 8,860,296	$ 8,100,257
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	7,093	23,182
Financial derivatives, at fair value	(26,599)	(58,152)
Derivative Credit Risk Valuation Adjustment, Derivative Assets	0	100
Derivative, Collateral [Abstract]
Collateral Pledged	24,926	25,643
Derivative Asset	7,093	23,182
Derivative Liability	1,673	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	2,086,347	1,642,609
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	5,240	18,508
Financial derivatives, at fair value	$ (5,990)	$ (18,909)
Derivative, Average Fixed Interest Rate	1.88%	1.73%
Derivative, Average Variable Interest Rate	1.40%	0.90%
Derivative, Average Remaining Maturity	5 years 5 months 17 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	$ 1,559,700
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	110
Financial derivatives, at fair value	$ (4,033)
Derivative, Average Fixed Interest Rate	1.38%
Derivative, Average Variable Interest Rate	1.45%
Derivative, Average Remaining Maturity	1 year 8 months 5 days
Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 365,500	$ 207,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	1,402	3,706
Financial derivatives, at fair value	$ (138)	$ (955)
Derivative, Average Fixed Interest Rate	2.16%	2.18%
Derivative, Average Variable Interest Rate	1.74%	1.11%
Derivative, Average Remaining Maturity	5 years 10 months 3 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	$ 16	$ 14
Derivative Credit Risk Valuation Adjustment, Derivative Liabilities	56	158
Not Designated as Hedging Instrument [Member] | Interest Rate Swap, Pay Fixed Non-Callable [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	345,333	435,827
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	339	339
Financial derivatives, at fair value	$ (16,352)	$ (32,951)
Derivative, Average Fixed Interest Rate	3.79%	4.06%
Derivative, Average Variable Interest Rate	1.40%	0.89%
Derivative, Average Remaining Maturity	6 years 8 months 5 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	$ 3,409,916	$ 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	1.24%	0.75%
Derivative, Average Variable Interest Rate	1.25%	0.74%
Derivative, Average Remaining Maturity	11 months	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	$ 1,053,500	$ 765,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	18	36
Financial derivatives, at fair value	$ (106)	$ (243)
Derivative, Average Variable Interest Rate	1.33%	0.78%
Derivative, Variable Interest Rate	1.42%	0.78%
Derivative, Average Remaining Maturity	10 months 28 days	10 months 15 days
Not Designated as Hedging Instrument [Member] | Future [Member]
Notional Disclosures [Abstract]
Derivative, Notional Amount	$ 40,000	$ 28,000
Derivative Asset, Fair Value, Amount Not Offset Against Collateral [Abstract]
Financial derivatives, at fair value	0	0
Financial derivatives, at fair value	$ (36)	$ (155)
Derivative, Weighted-Average Forward Price	$ 123.96	$ 123.73