Summary of Effects of Adopting Topic 606 on Condensed Consolidated Balance Sheet (Detail) - USD ($)	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Deferred revenue	$ 3,491,000		$ 1,090,000
Accumulated deficit	$ (404,983,000)		$ (399,647,000)
Topic 606
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Deferred revenue		$ 1,088,000
Accumulated deficit		(399,645,000)
Topic 606 | Adjustments
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Deferred revenue		(2,000)
Accumulated deficit		$ 2,000