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, 2019	Dec. 31, 2018
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Total beginning balance	$ 547,484	$ 487,094
Total ending balance		547,484
Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Total beginning balance	111,586	35,055
Total ending balance		111,586
Defined Benefit Plan, Hedged Funds and Other Alternative Strategies | Level 3
Defined Benefit Plan, Change in Fair Value of Plan Assets, Level 3 Reconciliation [Roll Forward]
Total beginning balance	35,029	37,950
Relating to assets held at the reporting date	20,811	(2,921)
Purchases, sales and settlements	55,746
Total ending balance	$ 111,586	$ 35,029