Summary of Significant Accounting Policies - Schedule of Adjustments to Consolidated Balance Sheet Due to Adoption of New Guidance (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Item Effected [Line Items]
Current portion of deferred revenue	$ 14,268		$ 21,188
Deferred revenue, net of current portion	31,996		44,111
Accumulated deficit	(181,959)		$ (121,113)
Accounting Standards Update (“ASU”) No. 2014-09 [Member]
Item Effected [Line Items]
Current portion of deferred revenue		$ 18,419
Deferred revenue, net of current portion		41,449
Accumulated deficit		(115,682)
Accounting Standards Update (“ASU”) No. 2014-09 [Member] | ASC 606 Adjustment [Member]
Item Effected [Line Items]
Current portion of deferred revenue	(2,915)	(2,769)
Deferred revenue, net of current portion	(1,612)	(2,662)
Accumulated deficit	$ 4,527	$ 5,431