Summary of Significant Accounting Policies - Summary of Accounts Receivable Allowance Activity (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Accounting Policies [Abstract]
Balance at beginning of year	$ 37	$ 263	$ 358
Additions	78
Deductions		(226)	(95)
Balance at end of year	$ 115	$ 37	$ 263