Collaboration Revenue - Comparison of Reported Condensed Consolidated Balance Sheet to Pro-forma Amounts (Detail) - USD ($) $ in Thousands	Sep. 30, 2018	Jan. 01, 2018	Dec. 31, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party	$ 17,901		$ 12,079
Deferred revenue, net of current portion - related party	89,554		84,847
Accumulated deficit	(368,083)		$ (263,571)
ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party		$ 17,418
Deferred revenue, net of current portion - related party		106,365
Accumulated deficit		$ (290,428)
Previous Accounting in Effect [Member] | ASU 2014-09 [Member] | Pro Forma [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue - related party	12,118
Deferred revenue, net of current portion - related party	76,034
Accumulated deficit	$ (348,780)