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		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue	$ 11,528	$ 7,711	$ 35,336	$ 31,259
Research and development expense	116,744	61,545	328,867	180,464
Net loss	$ (145,480)	$ (78,805)	$ (406,602)	$ (218,415)
Net loss per share - basic and diluted:	$ (2.73)	$ (1.73)	$ (7.95)	$ (5.14)
Collaboration [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue	$ 10,926	$ 5,211	$ 33,971	$ 18,189
ASU 2014-09 [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Research and development expense	(2,789)		(4,589)
Net loss	$ 4,886		$ 13,705
Net loss per share - basic and diluted:	$ 0.09		$ 0.27
ASU 2014-09 [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Research and development expense	$ 119,533		$ 333,456
Net loss	$ (150,366)		$ (420,307)
Net loss per share - basic and diluted:	$ (2.82)		$ (8.22)
ASU 2014-09 [Member] | Collaboration [Member] | Adjustments [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue	$ 2,097		$ 9,116
ASU 2014-09 [Member] | Collaboration [Member] | Balances Without Adoption of Topic 606 [Member]
Basis Of Presentation And Significant Accounting Policies [Line Items]
Revenue	$ 8,829		$ 24,855