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Retirement Plans - Net Periodic Benefit Costs and Benefit Payments (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2020
Dec. 31, 2019
Dec. 31, 2018
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost $ 1,295 $ 1,278  
Interest cost 16,643 22,408  
Defined Benefit Plan, Expected Future Benefit Payment [Abstract]      
2021 42,436    
2022 42,590    
2023 42,611    
2024 42,440    
2025 42,329    
2026-2030 206,549    
Defined Benefit Pension Plans      
Defined Benefit Plan, Net Periodic Benefit Cost (Credit) [Abstract]      
Service cost 1,295 1,278 $ 1,395
Interest cost 16,643 22,376 20,933
Expected return on assets (28,016) (29,654) (34,267)
Amortization of actuarial loss 10,804 10,837 10,744
Net periodic benefit cost (credit) $ 726 $ 4,837 $ (1,195)