Derivative Financial Instruments Offsetting of Derivative Assets and Liabilities (Details) - USD ($) $ in Thousands	Jun. 30, 2017	Dec. 31, 2016
Offsetting of Derivative Assets and Liabilities [Abstract]
Derivative Asset, Fair Value, Gross Asset	$ 168,156	$ 122,096
Derivative Liability, Fair Value, Gross Liability	(106,211)	(213,804)
Derivative Asset, Not Offset, Policy Election Deduction	(64,628)	(118,080)
Derivative Liability, Not Offset, Policy Election Deduction	64,628	118,080
Derivative Asset, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	103,528	4,016
Derivative Liability, Fair Value, Offset Against Collateral, Net of Not Subject to Master Netting Arrangement, Policy Election	$ (41,583)	$ (95,724)