Significant Concentrations - Summary of Rebate Program Recorded Under Rebate Credits (Detail) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016
Cost of revenue [Member]
Concentration Risk [Line Items]
Earnings under the rebate program	$ 499	$ 345
Reductions to marketing expense [Member]
Concentration Risk [Line Items]
Earnings under the rebate program	$ 658	$ 1,085

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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