Commitments and contingencies - Other Agreements - (Details) - Non-profit organization agreement $ in Millions	12 Months Ended
Dec. 31, 2016 USD ($) multiple	Dec. 31, 2017 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Maximum funding	$ 4.0
Funding multiple \| multiple	2.6
Milestone payment liability		$ 1.0

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration