Benefit Plans - Change in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Pension and Supplemental Executive Retirement Plans
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets at Beginning of Year	$ 402,691	$ 359,719
Company Contributions	12,453	10,205
Plan Participants' Contributions	0	0
Benefit Payments from the Fund	(40,260)	(30,829)
Benefit Payments paid directly by Company	(5,953)	(3,105)
Actual Return on Assets	42,314	70,262
Other Adjustments	0	(3,561)
Fair Value of Plan Assets at End of Year	411,245	402,691
Other Postretirement Benefit Plan
Defined Benefit Plan, Change in Fair Value of Plan Assets [Roll Forward]
Fair Value of Plan Assets at Beginning of Year	99,590	77,762
Company Contributions	0	0
Plan Participants' Contributions	425	382
Benefit Payments from the Fund	(1,975)	(826)
Benefit Payments paid directly by Company	0	0
Actual Return on Assets	21,409	22,654
Other Adjustments	(425)	(382)
Fair Value of Plan Assets at End of Year	$ 119,024	$ 99,590