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Pensions and Other Post-retirement Benefits - Additional Information (Detail) - USD ($) $ in Millions | 12 Months Ended
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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) |

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