Benefit Plans - Development of Funded Status (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]
Accumulated Benefit Obligation	$ 412,939	$ 375,562
Projected Benefit Obligation	(413,350)	(376,153)	$ (417,770)
Plan Assets at Fair Value	402,691	359,719	401,142
Funded Status - Overfunded/(Underfunded)	(10,659)	(16,434)
Other Postretirement Benefit Plan
Defined Benefit Plan Disclosure [Line Items]
Accumulated Benefit Obligation	27,496	28,085
Projected Benefit Obligation	(27,496)	(28,085)	(24,716)
Plan Assets at Fair Value	99,590	77,762	$ 85,303
Funded Status - Overfunded/(Underfunded)	$ 72,094	$ 49,677