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Pension Plans and Other Postretirement Benefit Plans - Components of Net Periodic Benefit Costs (Details) - USD ($)
$ in Thousands
 3 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Pension Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service cost $ 6,246 $ 5,546
Interest cost 6,580 6,971
Expected return on plan assets (9,775) (9,125)
Amortization of prior service cost 75 75
Net loss recognition 1,823 1,654
Net periodic benefit cost 4,949 5,121
Other Postretirement Benefits [Member]    
Defined Benefit Plan Disclosure [Line Items]    
Service cost 1,008 979
Interest cost 1,376 1,515
Expected return on plan assets (675) (630)
Amortization of prior service cost (275) (275)
Net loss recognition 1,173 1,243
Net periodic benefit cost $ 2,607 $ 2,832