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Retirement Benefit Obligations - Additional Information (Detail)
$ in Millions
3 Months Ended 6 Months Ended
Jun. 30, 2020
USD ($)
Jun. 30, 2020
USD ($)
Defined Benefit Plan Disclosure [Line Items]    
Contributions to benefit plan $ 0.0  
Other Postretirement Benefit Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Contributions to benefit plan 1.4  
Supplemental Employee Retirement Plans, Defined Benefit    
Defined Benefit Plan Disclosure [Line Items]    
Benefit payments under SERP Plan   $ 0.3
Expected additional benefit payments for the remainder of 2019 $ 0.4 $ 0.4