Accounts Receivable - 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	$ 150	$ 150	$ 150
Provisions for doubtful accounts	30	104	67
Charge-offs, net of recoveries	(30)	(104)	(67)
Balance at end of year	$ 150	$ 150	$ 150