Commitments and contingencies - Other Agreements - (Details) $ in Thousands	9 Months Ended	12 Months Ended	45 Months Ended
	Sep. 30, 2019 USD ($)	Dec. 31, 2016 USD ($) multiple	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Sep. 30, 2016 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Non-current liability	$ 32,799		$ 32,799	$ 6,711
Various contract research organizations and institutions
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 100
Various contract research organizations and institutions | Minimum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of days written notice required by reporting entity to terminate agreement			30 days
Various contract research organizations and institutions | Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Number of days written notice required by reporting entity to terminate agreement			90 days
Non-profit organization agreement
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum funding		$ 4,000
Funding multiple | multiple		2.6
Milestone payment liability					$ 1,000
Non-current liability					$ 1,000
MRIC
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Amount of investment						$ 2,000