Benefit Plans - Change in Project Benefit/Accumulated Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation at Beginning of Year	$ 376,153	$ 417,770
Company Service Cost	8,345	10,530	$ 9,556
Interest Cost	15,705	15,095	15,475
Plan Participants' Contributions	0	0
Net Actuarial (Gain)/Loss due to Assumption Changes	43,302	(36,132)
Net Actuarial (Gain)/Loss due to Plan Experience	3,811	2,487
Benefit Payments from Fund	(30,829)	(32,674)
Benefit Payments Directly by Company	(3,105)	(908)
Plan Amendments	(5)	(15)
Other Adjustment	0	0
Settlement (Gain)Loss	(27)	0
Benefit Obligation at End of Year	413,350	376,153	417,770
Other Postretirement Benefit Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation at Beginning of Year	28,085	24,716
Company Service Cost	1,345	1,160	813
Interest Cost	1,130	834	706
Plan Participants' Contributions	382	475
Net Actuarial (Gain)/Loss due to Assumption Changes	(1,215)	(1,209)
Net Actuarial (Gain)/Loss due to Plan Experience	(860)	(692)
Benefit Payments from Fund	(826)	(1,077)
Benefit Payments Directly by Company	0	0
Plan Amendments	0	3,928
Other Adjustment	(545)	(50)
Settlement (Gain)Loss	0	0
Benefit Obligation at End of Year	$ 27,496	$ 28,085	$ 24,716