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Retirement Benefit Obligations - Additional Information (Detail)
$ in Millions
3 Months Ended
Mar. 31, 2019
USD ($)
Pension Plans  
Defined Benefit Plan Disclosure [Line Items]  
Contributions to benefit plan $ 1.3
Other Postretirement Benefit Plans, Defined Benefit  
Defined Benefit Plan Disclosure [Line Items]  
Contributions to benefit plan 0.4
Supplemental Employee Retirement Plans, Defined Benefit  
Defined Benefit Plan Disclosure [Line Items]  
Benefit payments under SERP Plan 0.1
Expected additional benefit payments for the remainder of 2018 $ 0.5