Summary of Operations and Significant Accounting Policies - Summary of Allowance for Doubtful Accounts (Detail) - USD ($)	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Receivables [Abstract]
Allowance for doubtful accounts, beginning balance	$ 78,000	$ 268,000	$ 291,000
Charges to expense	192,000	(115,000)	116,000
Write-offs of uncollectible accounts, net of recoveries	(5,000)	(75,000)	(139,000)
Allowance for doubtful accounts, ending balance	$ 265,000	$ 78,000	$ 268,000