Derivative Instruments Derivative Instruments (Balance Sheet Offsetting) (Details) - USD ($) $ in Thousands	Sep. 30, 2019	Dec. 31, 2018
Derivative Asset, Fair Value, Gross Asset Including Not Subject to Master Netting Arrangement	$ 134,874	$ 41,210
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Assets	134,874	41,210
Derivative Liability, Fair Value, Gross Liability Including Not Subject to Master Netting Arrangement	11,756	13,421
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liabilities	11,756	13,421
Derivative Subject to Master Netting Agreement [Member]
Derivative Asset, Fair Value, Gross Asset	133,971	40,613
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Net Amount Presented in the Balance Sheet	133,971	40,613
Derivative Asset, Not Offset, Policy Election Deduction	(10,081)	(2,992)
derivatve asset, fair value of collateral	(75,373)	(26,174)
Derivative Asset, Fair Value, Amount Offset Against Collateral	48,517	11,447
Derivative Liability, Fair Value, Gross Liability	11,555	13,333
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Net Amount Presented in the Balance Sheet	11,555	13,333
Derivative Liability, Not Offset, Policy Election Deduction	(10,081)	(2,992)
Derivative Liability, Fair Value of Collateral	(533)	(261)
Derivative Liability, Fair Value, Amount Offset Against Collateral	941	10,080
Derivative Not Subject to Master Netting Agreement [Member]
Derivative Asset, Fair Value, Gross Asset	903	597
Derivative Asset, Amount Offset by Liabiilty	0	0
Derivative Asset, Not Subject to Master Netting Arrangement	903	597
Derivative Liability, Fair Value, Gross Liability	201	88
Derivative Liability, Amount Offset by Asset	0	0
Derivative Liability, Not Subject to Master Netting Arrangement	$ 201	$ 88