Pension Plans and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Pension Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	$ 6,412	$ 5,549	$ 18,912	$ 16,645
Interest cost	6,528	6,968	19,638	20,906
Expected return on plan assets	(9,835)	(8,625)	(29,314)	(26,375)
Amortization of prior service cost	75	75	225	225
Net loss recognition	1,592	1,678	5,103	5,011
Net periodic benefit cost	4,772	5,645	14,564	16,412
Other Postretirement Benefits [Member]
Defined Benefit Plan Disclosure [Line Items]
Service cost	1,062	982	2,967	2,941
Interest cost	1,305	1,524	3,990	4,563
Expected return on plan assets	(509)	(590)	(1,967)	(1,810)
Amortization of prior service cost	(275)	(275)	(825)	(825)
Net loss recognition	777	1,243	3,196	3,729
Net periodic benefit cost	$ 2,360	$ 2,884	$ 7,361	$ 8,598