Benefit Plans - Change in Project Benefit/Accumulated Benefit Obligations (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation at Beginning of Year	$ 413,350	$ 376,153
Company Service Cost	7,342	8,345	$ 10,530
Interest Cost	13,036	15,705	15,095
Plan Participants' Contributions	0	0
Net Actuarial (Gain)/Loss	36,196	47,113
Benefit Payments from the Fund	(40,260)	(30,829)
Benefit Payments Directly by Company	(5,953)	(3,105)
Plan Amendments	2	(5)
Other Adjustment	0	0
Settlement (Gain)Loss	0	(27)
Benefit Obligation at End of Year	423,713	413,350	376,153
Other Postretirement Benefit Plan
Defined Benefit Plan, Change in Benefit Obligation [Roll Forward]
Benefit Obligation at Beginning of Year	27,496	28,085
Company Service Cost	1,263	1,345	1,160
Interest Cost	832	1,130	834
Plan Participants' Contributions	425	382
Net Actuarial (Gain)/Loss	660	(2,075)
Benefit Payments from the Fund	(1,975)	(826)
Benefit Payments Directly by Company	0	0
Plan Amendments	0	0
Other Adjustment	13	(545)
Settlement (Gain)Loss	0	0
Benefit Obligation at End of Year	$ 28,714	$ 27,496	$ 28,085