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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, 2020
Sep. 30, 2019
Sep. 30, 2020
Sep. 30, 2019
Pension Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 5,549 $ 4,948 $ 16,645 $ 14,770
Interest cost 6,968 7,134 20,906 21,372
Expected return on plan assets (8,625) (7,913) (26,375) (23,681)
Amortization of prior service cost 75 75 225 225
Net loss recognition 1,678 2,553 5,011 7,394
Net periodic benefit cost 5,645 6,797 16,412 20,080
Other Postretirement Benefits [Member]        
Defined Benefit Plan Disclosure [Line Items]        
Service cost 982 754 2,941 2,279
Interest cost 1,524 1,140 4,563 3,676
Expected return on plan assets (590) (616) (1,810) (1,945)
Amortization of prior service cost (275) (275) (825) (825)
Net loss recognition 1,243 1,299 3,729 3,874
Net periodic benefit cost $ 2,884 $ 2,302 $ 8,598 $ 7,059