Summary of significant accounting policies (Details) $ / shares in Units, $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2018 USD ($) $ / shares	Sep. 30, 2017 USD ($) $ / shares	Sep. 30, 2018 USD ($) item $ / shares	Sep. 30, 2017 USD ($) $ / shares	Dec. 31, 2017 USD ($)
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Number of open contracts | item			1
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	$ 2,094		$ 5,611
Loss before income taxes	(20,289)	$ (23,346)	(65,937)	$ (58,900)
Net loss	$ (20,289)	$ (23,346)	$ (65,757)	$ (58,869)
Net loss per share, basic and diluted | $ / shares	$ (0.63)	$ (0.89)	$ (2.06)	$ (2.27)
Condensed Consolidated Balance Sheets
Deferred revenue, current	$ 20,250		$ 20,250		$ 3,380
Deferred revenue, non-current	94,804		94,804		28,180
Accumulated deficit	(246,520)		(246,520)		$ (160,713)
Condensed Consolidated Statement of Cash Flows
Net loss			(65,757)	$ (58,869)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue			63,388	$ (3,790)
ASU 2014-09 | As recognized under Topic 605
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	3,054		7,267
Loss before income taxes	(19,329)		(64,281)
Net loss	$ (19,329)		$ (64,101)
Net loss per share, basic and diluted | $ / shares	$ (0.60)		$ (2.00)
Condensed Consolidated Balance Sheets
Deferred revenue, current	$ 19,078		$ 19,078
Deferred revenue, non-current	74,390		74,390
Accumulated deficit	(224,934)		(224,934)
Condensed Consolidated Statement of Cash Flows
Net loss			(64,101)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue			61,732
ASU 2014-09 | Effect of change
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	(960)		(1,656)
Loss before income taxes	(960)		(1,656)
Net loss	$ (960)		$ (1,656)
Net loss per share, basic and diluted | $ / shares	$ (0.03)		$ (0.06)
Condensed Consolidated Balance Sheets
Deferred revenue, current	$ 1,172		$ 1,172
Deferred revenue, non-current	20,414		20,414
Accumulated deficit	$ (21,586)		(21,586)
Condensed Consolidated Statement of Cash Flows
Net loss			(1,656)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue			$ 1,656