Significant Risk Concentrations - Summary of Rebate Program Recorded Under Rebate Credits (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Cost of revenue [Member]
Concentration Risk [Line Items]
Earnings under the rebate program	$ 260	$ 110
Reduction to marketing expense [Member]
Concentration Risk [Line Items]
Earnings under the rebate program	$ 266	$ 155

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.

Name:	us-gaap_ConcentrationRiskLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration