Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) - USD ($) $ in Thousands	Mar. 31, 2018	Dec. 31, 2017
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	$ 5,092	$ 8,349
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Assets	5,092	8,349
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	16,373	8,074
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liabilities	16,373	8,074
Derivative Subject to Master Netting Agreement [Member]
Derivative Asset, Fair Value, Gross Asset	4,471	7,726
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Net Amount Presented in the Balance Sheet	4,471	7,726
Derivative Asset, Not Offset, Policy Election Deduction	(451)	(233)
derivatve asset, fair value of collateral	(1,505)	(824)
Derivative Asset, Fair Value, Amount Offset Against Collateral	2,515	6,669
Derivative Liability, Fair Value, Gross Liability	16,237	7,935
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Net Amount Presented in the Balance Sheet	16,237	7,935
Derivative Liability, Not Offset, Policy Election Deduction	(451)	(233)
Derivative Liability, Fair Value of Collateral	(1,188)	(1,570)
Derivative Liability, Fair Value, Amount Offset Against Collateral	14,598	6,132
Derivative Not Subject to Master Netting Agreement [Member]
Derivative Asset, Fair Value, Gross Asset	621	623
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Not Subject to Master Netting Arrangement	621	623
Derivative Liability, Fair Value, Gross Liability	136	139
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Not Subject to Master Netting Arrangement	$ 136	$ 139