Commitments and contingencies - Other Agreements - (Details) $ in Millions	6 Months Ended	12 Months Ended
	Jun. 30, 2020 USD ($)	Dec. 31, 2016 USD ($) multiple	Dec. 31, 2017 USD ($)
Various contract research organizations and institutions
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Milestone payable upon achievement	$ 0.1
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		$ 4.0
Funding multiple | multiple		2.6
Milestone payment liability			$ 1.0