Basis of presentation, principles of consolidation and significant accounting policies - Additional Information (Detail) $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) Segment	Jan. 01, 2018 USD ($)	Dec. 31, 2017 USD ($)
Basis Of Presentation And Significant Accounting Policies [Line Items]
Number of operating segment | Segment	1
Accumulated deficit	$ 1,058,308	$ 943,183	$ 913,808
Description of performance obligations	Under ASC 605, the Company accounted for contract modifications to the Celgene collaboration as they occurred and the accounting for those changes was prospective in nature. Through the application of the practical expedient discussed above in connection with the adoption of Topic 606, the Company reflected the aggregate effect of all modifications to the Celgene collaboration when identifying the satisfied and unsatisfied performance obligations, determining the transaction price, and allocating the transaction price. As a result, although the performance obligations identified under Topic 606 were generally consistent with the units of account identified under ASC 605, the timing of the allocation of the transaction price to the identified performance obligations under Topic 606 differed from the allocations of consideration under ASC 605. Accordingly, the transaction price ultimately allocated to each performance obligation under Topic 606 differed from the amounts allocated under ASC 605.
Deferred tax assets, valuation allowance	$ 7,900
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Accumulated deficit	$ 23,243	$ 29,375