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Retirement Benefit Obligations - Additional Information (Detail)
3 Months Ended
Mar. 31, 2017
USD ($)
Pension Plans  
Defined Benefit Plan Disclosure [Line Items]  
Contributions to benefit plan $ 1,400,000
Other Postretirement Benefit Plans, Defined Benefit  
Defined Benefit Plan Disclosure [Line Items]  
Contributions to benefit plan 400,000
Supplemental Employee Retirement Plans, Defined Benefit  
Defined Benefit Plan Disclosure [Line Items]  
Benefit payments under SERP Plan 8,500
Expected additional benefit payments under SERP Plan in 2016 $ 25,600