Benefit Plans - Development of Funded Status (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]
Accumulated Benefit Obligation	$ 423,305	$ 412,939
Projected Benefit Obligation	(423,713)	(413,350)	$ (376,153)
Plan Assets at Fair Value	411,245	402,691	359,719
Funded Status - Overfunded/(Underfunded)	(12,468)	(10,659)
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation	28,714	27,496
Projected Benefit Obligation	(28,714)	(27,496)	(28,085)
Plan Assets at Fair Value	119,024	99,590	$ 77,762
Funded Status - Overfunded/(Underfunded)	$ 90,310	$ 72,094