Benefit Plans - Change in Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2019	Dec. 31, 2018
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	$ 359,719	$ 401,142
Company Contributions	10,205	10,908
Plan Participants' Contributions	0	0
Benefit Payments from Fund	(30,829)	(32,674)
Benefit Payments paid directly by Company	(3,105)	(908)
Actual Return on Assets	70,262	(19,583)
Other Adjustments	(3,561)	834
Fair Value of Plan Assets at End of Year	402,691	359,719
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	77,762	85,303
Company Contributions	0	0
Plan Participants' Contributions	382	475
Benefit Payments from Fund	(826)	(1,077)
Benefit Payments paid directly by Company	0	0
Actual Return on Assets	22,654	(6,464)
Other Adjustments	(382)	(475)
Fair Value of Plan Assets at End of Year	$ 99,590	$ 77,762