Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) - USD ($) $ in Thousands	Mar. 31, 2019	Dec. 31, 2018
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	$ 51,691	$ 41,210
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Assets	51,691	41,210
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	10,300	13,421
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liabilities	10,300	13,421
Derivative Subject to Master Netting Agreement [Member]
Derivative Asset, Fair Value, Gross Asset	50,792	40,613
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Net Amount Presented in the Balance Sheet	50,792	40,613
Derivative Asset, Not Offset, Policy Election Deduction	(4,695)	(2,992)
derivatve asset, fair value of collateral	(28,385)	(26,174)
Derivative Asset, Fair Value, Amount Offset Against Collateral	17,712	11,447
Derivative Liability, Fair Value, Gross Liability	10,203	13,333
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Net Amount Presented in the Balance Sheet	10,203	13,333
Derivative Liability, Not Offset, Policy Election Deduction	(4,695)	(2,992)
Derivative Liability, Fair Value of Collateral	(199)	(261)
Derivative Liability, Fair Value, Amount Offset Against Collateral	5,309	10,080
Derivative Not Subject to Master Netting Agreement [Member]
Derivative Asset, Fair Value, Gross Asset	899	597
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Not Subject to Master Netting Arrangement	899	597
Derivative Liability, Fair Value, Gross Liability	97	88
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Not Subject to Master Netting Arrangement	$ 97	$ 88