Employee and Retiree Benefit Plans - Effects of Fair Value Measurements Using Significant Unobservable Inputs on Changes in Level 3 Plan Assets (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning balance	$ 547,484
Ending balance	586,720	$ 547,484
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning balance	111,586
Ending balance	97,685	111,586
Hedged Funds and Other Alternative Strategies | Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Beginning balance	111,586	35,029
Relating to assets held at the reporting date	5,694	20,811
Purchases, sales and settlements	(19,595)	55,746
Ending balance	$ 97,685	$ 111,586