Basis of presentation, principles of consolidation and significant accounting policies - Impact of Topic 606 Adoption on Condensed Consolidated Statements of Operations and Comprehensive Loss (Detail) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018	Mar. 31, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Collaboration revenue	$ 15,608	$ 6,832
Net loss	$ (115,126)	$ (68,712)
Net loss per share - basic and diluted:	$ (2.31)	$ (1.68)
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Collaboration revenue	$ 6,133
Net loss	$ 6,133
Net loss per share - basic and diluted:	$ 0.12
ASU 2014-09 [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Collaboration revenue	$ 9,475
Net loss	$ (121,259)
Net loss per share - basic and diluted:	$ (2.43)