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Pension and Other Postretirement Benefits - Plan assets and type of fair value measurement (Details) - Pension Benefits - USD ($)
$ in Thousands
Dec. 31, 2019
Dec. 31, 2018
Pension and Other Postretirement Benefits    
Fair value of plan assets $ 247,304 $ 216,744
Level 1    
Pension and Other Postretirement Benefits    
Fair value of plan assets 0 0
Level 2    
Pension and Other Postretirement Benefits    
Fair value of plan assets 1,682 1,682
Level 3    
Pension and Other Postretirement Benefits    
Fair value of plan assets 0 0
Fixed income    
Pension and Other Postretirement Benefits    
Fair value of plan assets 1,682 1,682
Fixed income | Level 1    
Pension and Other Postretirement Benefits    
Fair value of plan assets 0 0
Fixed income | Level 2    
Pension and Other Postretirement Benefits    
Fair value of plan assets 1,682 1,682
Fixed income | Level 3    
Pension and Other Postretirement Benefits    
Fair value of plan assets 0 0
Commingled funds | Measured at net asset value    
Pension and Other Postretirement Benefits    
Fair value of plan assets $ 245,622 $ 215,062