Benefit Plans - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Actuarial (Gain)/Loss	$ 103,861	$ 95,636
Net Prior Service Cost/(Credit)	(2,286)	(2,989)
Net Transition Obligation/(Asset)	0	0
Total at Year End	101,575	92,647	$ 89,390
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Actuarial (Gain)/Loss	(6,088)	(5,311)
Net Prior Service Cost/(Credit)	(11,991)	(18,640)
Net Transition Obligation/(Asset)	0	0
Total at Year End	$ (18,079)	$ (23,951)	$ (33,511)