Employee and Retiree Benefit Plans - Components of Net Periodic Benefit Expense (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Pension Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 7,967	$ 7,964	$ 8,994
Interest cost	21,127	27,835	26,168
Expected return on plan assets	(24,316)	(25,719)	(29,236)
Amortization of prior service cost (credit)	640	964	1,021
Recognized actuarial loss	22,828	14,106	21,893
Net periodic benefit expense	28,246	25,150	28,840
Termination benefits expense	8,434	0	0
Curtailment expense	586	0	0
Total net periodic benefit expense	37,266	25,150	28,840
Other Postretirement Benefits
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,373	1,559	1,965
Interest cost	2,626	3,864	3,427
Expected return on plan assets	0	0	0
Amortization of prior service cost (credit)	0	0	(38)
Recognized actuarial loss	(31)	(193)	0
Net periodic benefit expense	3,968	5,230	5,354
Termination benefits expense	0	0	0
Curtailment expense	(1,825)	0	0
Total net periodic benefit expense	$ 2,143	$ 5,230	$ 5,354