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Retirement Benefit Obligations - Additional Information (Detail)
9 Months Ended
Sep. 30, 2015
USD ($)
Pension Plans  
Defined Benefit Plan Disclosure [Line Items]  
Contributions to benefit plan $ 4,200,000
Other Postretirement Benefit Plans, Defined Benefit  
Defined Benefit Plan Disclosure [Line Items]  
Contributions to benefit plan 3,000,000
Supplemental Employee Retirement Plans, Defined Benefit  
Defined Benefit Plan Disclosure [Line Items]  
Contributions to benefit plan 8,500
Expected additional contribution to SERP plan in 2015 $ 31,800