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Pensions and Other Post-retirement Benefits - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Defined Benefit Plan Disclosure [Line Items]    
Percentage match of the first one percent of contributions made by participating employees 100.00%  
Percentage Match Of The Next Five Percent Of Contributions Made By Participating Employees 50.00%  
Medical and life insurance maximum participant coverage age, in years 65 years  
Decrease in the service and interest components for pension cost $ 7.7 $ 7.1
Other comprehensive gain(loss) as a result of after tax adjustments for additional minimum pension liability 12.1 $ (9.0)
Voluntary pension contribution $ 30.0  
Minimum    
Defined Benefit Plan Disclosure [Line Items]    
Target allocation to equity managers 45.00%  
Maximum    
Defined Benefit Plan Disclosure [Line Items]    
Target allocation to equity managers 55.00%  
Ten Year Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Number of historical years for which compounded annualized returns is considered 10 years  
Compounded annualized return of pension plan 6.30%  
Twenty Five Year Pension Plan    
Defined Benefit Plan Disclosure [Line Items]    
Number of historical years for which compounded annualized returns is considered 25 years  
Compounded annualized return of pension plan 9.20%  
Pension Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Estimated net actuarial gain that will be amortized from accumulated other comprehensive loss into net periodic benefit cost $ 18.7  
Estimated net prior service cost that will be amortized from accumulated other comprehensive loss into net periodic benefit cost (0.4)  
Post-Retirement Benefits    
Defined Benefit Plan Disclosure [Line Items]    
Estimated net actuarial gain that will be amortized from accumulated other comprehensive loss into net periodic benefit cost (0.1)  
Estimated net prior service cost that will be amortized from accumulated other comprehensive loss into net periodic benefit cost $ (0.4)