Benefit Plans - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Actuarial (Gain)/Loss	$ 99,826	$ 110,321
Net Prior Service Cost/(Credit)	(1,237)	(1,513)
Net Transition Obligation/(Asset)	0	0
Total at Year End	98,589	108,808	$ 108,054
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Actuarial (Gain)/Loss	(18,005)	939
Net Prior Service Cost/(Credit)	2,724	2,690
Net Transition Obligation/(Asset)	0	0
Total at Year End	$ (15,281)	$ 3,629	$ (15,576)