Benefit Plans - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Actuarial (Gain)/Loss	$ 110,321	$ 109,904
Net Prior Service Cost/(Credit)	(1,513)	(1,850)
Net Transition Obligation/(Asset)	0	0
Total at Year End	108,808	108,054	$ 101,575
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Actuarial (Gain)/Loss	939	(10,234)
Net Prior Service Cost/(Credit)	2,690	(5,342)
Net Transition Obligation/(Asset)	0	0
Total at Year End	$ 3,629	$ (15,576)	$ (18,079)