Certain Balance Sheet Accounts - Summary of Allowance for Doubtful Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Balance, beginning of year	$ 140	$ 153	$ 188
Provision for doubtful accounts	331	150	106
Write-offs	(316)	(163)	(141)
Balance, end of year	$ 155	$ 140	$ 153