Amortization Expense Included in Consolidated Statements of Operations (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016
Finite Lived Intangible Assets [Line Items]
Amortization expense	$ 1,455	$ 1,455
Cost of revenue
Finite Lived Intangible Assets [Line Items]
Amortization expense	448	448
Selling and Marketing Expense
Finite Lived Intangible Assets [Line Items]
Amortization expense	$ 1,007	$ 1,007