Description of Business and Summary of Significant Accounting Policies - Changes in Allowance for Doubtful Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Balance at beginning of year	$ 434	$ 377	$ 125
Provision, net of recoveries	1,674	648	411
Write-offs	1,396	591	159
Balance at end of year	$ 712	$ 434	$ 377