Summary of Allowance for Doubtful Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Financing Receivable, Allowance for Credit Losses [Line Items]
Balance, beginning of year	$ 801	$ 622	$ 703
Provision	1,345	190
Write-offs, net of recoveries	$ (2,146)	(11)	(81)
Balance, end of year		$ 801	$ 622