Employee and Retiree Benefit Plans - Projected Benefit Obligations, Accumulated Benefit Obligations and Fair Value of Plan Assets (Details) - USD ($) $ in Thousands	Dec. 31, 2020	Dec. 31, 2019
Funded plan | Qualified plan
Defined Benefit Plan Disclosure [Line Items]
Projected Benefit Obligations	$ 762,134	$ 688,249
Accumulated Benefit Obligations	753,475	676,177
Fair Value of Plan Assets	564,238	525,108
Unfunded plan | Other postretirement plans
Defined Benefit Plan Disclosure [Line Items]
Projected Benefit Obligations	108,378	108,401
Accumulated Benefit Obligations	108,378	108,401
Fair Value of Plan Assets	0	0
Unfunded plan | Nonqualified plan
Defined Benefit Plan Disclosure [Line Items]
Projected Benefit Obligations	201,372	177,999
Accumulated Benefit Obligations	198,792	171,934
Fair Value of Plan Assets	$ 0	$ 0