Benefit Plans - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Actuarial (Gain)/Loss	$ 109,904	$ 103,861
Net Prior Service Cost/(Credit)	(1,850)	(2,286)
Net Transition Obligation/(Asset)	0	0
Total at Year End	108,054	101,575	$ 92,647
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Actuarial (Gain)/Loss	(10,234)	(6,088)
Net Prior Service Cost/(Credit)	(5,342)	(11,991)
Net Transition Obligation/(Asset)	0	0
Total at Year End	$ (15,576)	$ (18,079)	$ (23,951)