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Accounts receivable, net - Allowance For Doubtful Accounts (Detail) - USD ($)
$ in Thousands
12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Receivables [Abstract]      
Beginning balance $ 790 $ 318 $ 105
Charges to expenses 2,020 772 213
Write-offs (958) (300)  
Ending balance $ 1,852 $ 790 $ 318