Collaboration Revenue - Comparison of Reported Condensed Consolidated Cash Flows to Pro-forma Amounts (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Net loss	$ (21,949)	$ (6,935)	$ (77,655)	$ (60,427)
Deferred revenue			(16,328)	$ (9,044)
Previous Accounting in Effect [Member] \| ASU 2014-09 [Member] \| Pro Forma [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Net loss	$ (27,549)		(85,209)
Deferred revenue			$ (8,774)

The portion of profit or loss for the period, net of income taxes, which is attributable to the parent.

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