Significant Risk Concentrations - Summary of Rebate Program Recorded Under Rebate Credits (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Cost of revenue [Member]
Concentration Risk [Line Items]
Earnings under the rebate program	$ 159	$ 126	$ 577	$ 323
Reduction to marketing expense [Member]
Concentration Risk [Line Items]
Earnings under the rebate program	$ 294	$ 185	$ 673	$ 558

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.

No definition available.

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