Amortization Expense Included in Condensed Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Finite Lived Intangible Assets [Line Items]
Amortization expense	$ 531	$ 364	$ 1,510	$ 1,092
Cost of revenue
Finite Lived Intangible Assets [Line Items]
Amortization expense	186	112	522	336
Selling and Marketing Expense
Finite Lived Intangible Assets [Line Items]
Amortization expense	$ 345	$ 252	$ 988	$ 756