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Pension and Other Postretirement Benefits - Additional Information (Details) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Defined Benefit Plan Disclosure [Line Items]      
Defined contribution expense $ 13,900 $ 13,500 $ 12,600
Accumulated benefit obligation 269,200 253,900  
Project benefit obligation for the pension plans with an accumulated benefit obligation in excess of plan assets 215,600 205,800  
Accumulated benefit obligation for the pension plans with an accumulated benefit obligation in excess of assets 212,700 203,100  
Fair value of plan assets for the pension plans with an accumulated benefit obligations in excess of plan assets 138,500 128,500  
Projected benefit obligation of plan assets 56,500 50,700  
Accumulated benefit obligation for the pension plans with an accumulated benefit obligation less than plan assets 56,500 50,700  
Fair value of plan assets for the pension plans with an accumulated benefit obligations less than plan assets $ 59,600 53,900  
Target asset allocation for the investment of the assets in fixed income securities minimum 75.00%    
Target asset allocation for the investment of the assets in fixed income securities maximum 80.00%    
Target asset allocation for the investment of the assets in equity securities minimum 20.00%    
Target asset allocation for the investment of the assets in equity securities maximum 25.00%    
Minimum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Target asset allocation for the investment of the assets in fixed income securities 30.00%    
Target asset allocation for the investment of the assets in equity securities 60.00%    
Maximum [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Target asset allocation for the investment of the assets in fixed income securities 40.00%    
Target asset allocation for the investment of the assets in equity securities 70.00%    
Pension Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Settlement loss (gain) $ (8) 7,630 128
Pension and other postretirement plans 4,600    
Other Benefits [Member]      
Defined Benefit Plan Disclosure [Line Items]      
Settlement loss (gain) 0 $ 0 $ 0
Pension and other postretirement plans $ 1,600