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Employee Benefit Plan - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Pension And Other Postretirement Benefit Expense [Abstract]      
Contributions to 401 (k) plan $ 0.2 $ 0.2 $ 0.2