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Accrued Expenses Payable - Additional Information (Detail) - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Accrued expenses [abstract]    
Current portion of provision for return condition $ 4.9 $ 2.3
Current portion of provision for maintenance $ 4.2 $ 3.5
Accrual expense settlement period 12 months