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Pension and Other Postretirement Benefit Plans - Summary of Reconciliation of Changes in Plans Benefit Obligations Fair Value of Assets and Funded Status (Details) - Pension Benefits - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Change in benefit obligation:      
Benefit obligation at beginning balance $ (463,436) $ (428,216)  
Interest cost (14,675) (18,324) $ (17,090)
Actuarial loss, net (53,831) (47,632)  
Benefits paid 18,476 30,736  
Lump sum settlement 44,450 0  
Benefit obligation at ending balance (469,016) (463,436) (428,216)
Change in plan assets:      
Fair value of assets, beginning balance 338,264 312,455  
Actual return on plan assets 55,215 54,945  
Employer contributions 14,700 1,600  
Benefits paid (18,476) (30,736)  
Lump sum settlement (44,450) 0  
Fair value of assets, ending balance 345,253 338,264 $ 312,455
Unfunded status at December 31 $ (123,763) $ (125,172)