Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) Customer	Sep. 30, 2017 Customer	Sep. 30, 2018 USD ($) Customer	Sep. 30, 2017 Customer	Dec. 31, 2017 USD ($)
Short-term investments	$ 26,900,000		$ 26,900,000		$ 42,900,000
Subscription contracts			The Company’s subscription contracts are generally two years or longer in length, billed annually in advance, and non-cancelable.
Incremental and Recoverable Costs
Deferred cost amortization period			4 years
Money Market Funds and U.S. Government Securities
Cash and cash equivalents fair value disclosure	$ 47,800,000		$ 47,800,000
Customer Concentration Risk | Revenue
Number of customers above 10% threshold | Customer	0	0	0	0
Customer Concentration Risk | Accounts Receivable
Number of customers above 10% threshold | Customer	0	0	0	0
Maximum
Cash and cash equivalent, FDIC insured amount	$ 250,000		$ 250,000