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Pension and Other Postretirement Benefit Plans - Summary of Net Period Benefit Costs (Details) - Pension Benefits - USD ($)
$ in Thousands
 12 Months Ended
Aug. 31, 2020
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan And Other Postretirement Benefit Plan Table Text Block [Line Items]        
Interest cost   $ 14,675 $ 18,324 $ 17,090
Expected return on plan assets   (15,420) (18,510) (18,790)
Amortization of prior service credit   (1,432) (1,432) (1,432)
Amortization of actuarial loss   8,622 6,516 7,362
Net periodic benefit   6,445 4,898 4,230
Settlement charge   18,071 0 0
Net cost   $ 24,516 $ 4,898 $ 4,230
Weighted-average discount rate used to measure benefit obligations   2.60% 3.53% 4.41%
Weighted average assumptions used to determine net benefit cost:        
Discount rate   3.53% 4.41% 3.81%
Expected return on plan assets   5.00% 5.75% 5.75%
Discount rate due to settlement charge 2.76%