Commitments and contingencies - Other Agreements - (Details) $ in Thousands	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2016 USD ($) multiple	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2017 USD ($)	Dec. 31, 2017 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expenses incurred		$ 23,241	$ 24,503	$ 73,545	$ 63,305
Non-profit organization agreement
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Per Milestone, maximum milestone payments to be received from collaborative partner						$ 1,000
Deferred revenue		1,000		1,000
Maximum funding	$ 4,000
Funding multiple | multiple	2.6
MRIC
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Expenses incurred		$ 100		$ 200