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Financial Statement Details - Additional Information (Detail) - USD ($)
$ in Millions
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Organization, Consolidation and Presentation of Financial Statements [Abstract]      
Vehicles and equipment under capital leases, net $ 1.6 $ 1.8  
Vehicles and equipment under capital leases, accumulated depreciation (1.5) (1.1)  
Depreciation and amortization expense $ 8.0 $ 7.8 $ 5.0