Summary of significant accounting policies - Antidilutive securities (Details) - shares	9 Months Ended
Sep. 30, 2019	Sep. 30, 2018
Anti-dilutive securities
Anti-dilutive securities	5,205,407	3,772,327
Warrants
Anti-dilutive securities
Anti-dilutive securities	39,474	110,365
Restricted stock units
Anti-dilutive securities
Anti-dilutive securities	447,336
Stock options.
Anti-dilutive securities
Anti-dilutive securities	4,718,597	3,661,962

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