Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) - USD ($) $ in Thousands	Jun. 30, 2019	Dec. 31, 2018
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	$ 106,430	$ 41,210
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Assets	106,430	41,210
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	10,012	13,421
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liabilities	10,012	13,421
Derivative Subject to Master Netting Agreement [Member]
Derivative Asset, Fair Value, Gross Asset	105,389	40,613
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Net Amount Presented in the Balance Sheet	105,389	40,613
Derivative Asset, Not Offset, Policy Election Deduction	(7,837)	(2,992)
derivatve asset, fair value of collateral	(60,437)	(26,174)
Derivative Asset, Fair Value, Amount Offset Against Collateral	37,115	11,447
Derivative Liability, Fair Value, Gross Liability	9,803	13,333
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Net Amount Presented in the Balance Sheet	9,803	13,333
Derivative Liability, Not Offset, Policy Election Deduction	(7,837)	(2,992)
Derivative Liability, Fair Value of Collateral	(336)	(261)
Derivative Liability, Fair Value, Amount Offset Against Collateral	1,630	10,080
Derivative Not Subject to Master Netting Agreement [Member]
Derivative Asset, Fair Value, Gross Asset	1,041	597
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Not Subject to Master Netting Arrangement	1,041	597
Derivative Liability, Fair Value, Gross Liability	209	88
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Not Subject to Master Netting Arrangement	$ 209	$ 88