Allowance for Doubtful Accounts Receivable and Sales Returns (Additional Information) (Detail) $ in Thousands	3 Months Ended							12 Months Ended
	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Mar. 31, 2017 USD ($)	Dec. 31, 2018 USD ($) Customer	Dec. 31, 2017 USD ($) Customer	Dec. 31, 2016 USD ($) Customer
Concentration Risk [Line Items]
Net revenue | $	$ 17,579	$ 15,724	$ 19,650	$ 18,865	$ 18,557	$ 21,605	$ 19,691
Accounts Receivable
Concentration Risk [Line Items]
Number of major customers								3
Percentage by major customer								10.00%	20.00%
Accounts Receivable | Company A
Concentration Risk [Line Items]
Number of major customers									1
Percentage by major customer								23.00%
Accounts Receivable | Company B
Concentration Risk [Line Items]
Percentage by major customer								11.00%
Accounts Receivable | Company C
Concentration Risk [Line Items]
Percentage by major customer								10.00%
Mobile Services | Revenue by Segment
Concentration Risk [Line Items]
Number of major customers								1	1	1
Percentage by major customer								11.00%	10.00%	11.00%
Net revenue | $								$ 7,700	$ 8,000	$ 9,100