Net loss per share (Details) - shares	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018
Warrants \| Series C Redeemable Convertible Preferred Stock Warrants (as converted)
Net loss per share
Potentially dilutive shares excluded in the computation of diluted net loss per share	70,833	70,833
Stock options outstanding
Net loss per share
Potentially dilutive shares excluded in the computation of diluted net loss per share	4,528,342	3,576,062
RSUs outstanding
Net loss per share
Potentially dilutive shares excluded in the computation of diluted net loss per share	55,626	159,064

Securities (including those issuable pursuant to contingent stock agreements) that could potentially dilute basic earnings per share (EPS) or earnings per unit (EPU) in the future that were not included in the computation of diluted EPS or EPU because to do so would increase EPS or EPU amounts or decrease loss per share or unit amounts for the period presented.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 260
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6371337&loc=d3e3550-109257

Name:	us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration