Summary of Significant Accounting Policies - Allowance for Doubtful Accounts (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Allowance for Doubtful Accounts Receivable [Roll Forward]
Beginning Balance	$ 1,958	$ 1,263	$ 677
Provision for Credit Losses	906	1,494	915
Charge-offs	(2,459)	(799)	(329)
Ending Balance	$ 405	$ 1,958	$ 1,263