Summary of significant accounting policies (Details) $ / shares in Units, $ in Thousands	3 Months Ended
	Mar. 31, 2018 USD ($) item $ / shares	Mar. 31, 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	$ 942
Loss from operations	(20,106)	$ (16,874)
Net loss	$ (19,926)	$ (16,648)
Net loss per share, basic and diluted | $ / shares	$ (0.63)	$ (0.65)
Condensed Consolidated Balance Sheets
Deferred revenue, current	$ 20,217		$ 3,380
Deferred revenue, non-current	99,506		28,180
Accumulated deficit	(200,689)		$ (160,713)
Condensed Consolidated Statement of Cash Flows
Net loss	(19,926)	$ (16,648)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue	68,057	(1,464)
Cash, cash equivalents, and restricted cash beginning of period	32,265	37,376
Cash, cash equivalents, and restricted cash end of period	141,106	$ 44,339
ASU 2014-09 | As recognized under Topic 605
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	1,305
Loss from operations	(19,743)
Net loss	$ (19,563)
Net loss per share, basic and diluted | $ / shares	$ (0.62)
Condensed Consolidated Balance Sheets
Deferred revenue, current	$ 18,967
Deferred revenue, non-current	80,463
Accumulated deficit	(180,396)
Condensed Consolidated Statement of Cash Flows
Net loss	(19,563)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue	67,694
ASU 2014-09 | Effect of change
Condensed Consolidated Statements of Operations and Comprehensive Loss
Collaboration revenue	(363)
Loss from operations	(363)
Net loss	$ (363)
Net loss per share, basic and diluted | $ / shares	$ (0.01)
Condensed Consolidated Balance Sheets
Deferred revenue, current	$ 1,250
Deferred revenue, non-current	19,043
Accumulated deficit	(20,293)
Condensed Consolidated Statement of Cash Flows
Net loss	(363)
Adjustments to reconcile net loss to net cash provided by operating activities:
Deferred revenue	$ 363