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Retirement Benefit Obligations - Additional Information (Detail)
6 Months Ended
Jun. 30, 2015
USD ($)
Pension Plans  
Defined Benefit Plan Disclosure [Line Items]  
Contributions to benefit plan $ 1,800,000
Other Postretirement Benefit Plans, Defined Benefit  
Defined Benefit Plan Disclosure [Line Items]  
Contributions to benefit plan 0
Supplemental Employee Retirement Plans, Defined Benefit  
Defined Benefit Plan Disclosure [Line Items]  
Contributions to benefit plan 17,000
Expected additional contribution to SERP plan $ 23,300