Pension Plans and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,254	$ 5,550	$ 12,500	$ 11,096
Interest cost	6,530	6,967	13,110	13,938
Expected return on plan assets	(9,704)	(8,625)	(19,479)	(17,750)
Amortization of prior service cost	75	75	150	150
Net loss recognition	1,688	1,679	3,511	3,333
Net periodic benefit cost	4,843	5,646	9,792	10,767
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	897	980	1,905	1,959
Interest cost	1,309	1,524	2,685	3,039
Expected return on plan assets	(783)	(590)	(1,458)	(1,220)
Amortization of prior service cost	(275)	(275)	(550)	(550)
Net loss recognition	1,246	1,243	2,419	2,486
Net periodic benefit cost	$ 2,394	$ 2,882	$ 5,001	$ 5,714