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Property and equipment - Summary of Depreciation Expense (Detail) - USD ($)
$ in Thousands
 12 Months Ended
Dec. 31, 2017
Dec. 31, 2016
Dec. 31, 2015
Property, Plant and Equipment [Line Items]      
Depreciation expense $ 7,948 $ 6,165 $ 5,646
Cost of Revenue [Member]      
Property, Plant and Equipment [Line Items]      
Depreciation expense 7,647 5,848 4,873
General and administrative expenses [Member]      
Property, Plant and Equipment [Line Items]      
Depreciation expense 277 306 628
Sales and marketing expenses [Member]      
Property, Plant and Equipment [Line Items]      
Depreciation expense 0 5 7
Research and Development Expense [Member]      
Property, Plant and Equipment [Line Items]      
Depreciation expense $ 24 $ 6 $ 8