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Employee Benefit Plans - Components of Net Periodic Benefit Cost (Details) - USD ($)
$ in Thousands
 3 Months Ended 6 Months Ended
Jun. 30, 2021
Jun. 30, 2020
Jun. 30, 2021
Jun. 30, 2020
Defined Benefit Pension Plans        
Defined Benefit Plan Disclosure [Line Items]        
Service cost $ 1,293 $ 1,112 $ 2,586 $ 2,221
Interest cost 2,530 3,737 5,070 7,518
Expected return on plan assets (4,637) (4,902) (9,264) (9,823)
Amortization of prior service cost (credit) 119 43 230 86
Recognized actuarial loss (gain) 2,775 2,356 5,560 4,715
Net periodic benefit cost (credit) 2,080 2,346 4,182 4,717
Postretirement Benefit Plans        
Defined Benefit Plan Disclosure [Line Items]        
Interest cost 19 35 39 70
Amortization of prior service cost (credit) (509) (509) (1,019) (1,018)
Recognized actuarial loss (gain) (2) (19) (5) (38)
Net periodic benefit cost (credit) $ (492) $ (493) $ (985) $ (986)