Benefit Plans - Accumulated Other Comprehensive Income (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan Disclosure [Line Items]
Net Actuarial (Gain)/Loss	$ 98,899	$ 99,826
Net Prior Service Cost/(Credit)	(988)	(1,237)
Net Transition Obligation/(Asset)	0	0
Total at Year End	97,911	98,589	$ 108,808
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Net Actuarial (Gain)/Loss	(30,565)	(18,005)
Net Prior Service Cost/(Credit)	2,673	2,724
Net Transition Obligation/(Asset)	0	0
Total at Year End	$ (27,892)	$ (15,281)	$ 3,629