Summary of significant accounting policies - Antidilutive securities (Details) - shares	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Anti-dilutive securities
Anti-dilutive securities	4,016,210	19,567,011
Series A-1 convertible preferred stock
Anti-dilutive securities
Anti-dilutive securities		5,574,467
Series B-1 convertible preferred stock
Anti-dilutive securities
Anti-dilutive securities		7,319,307
Series C-1 convertible preferred stock
Anti-dilutive securities
Anti-dilutive securities		3,260,897
Warrants
Anti-dilutive securities
Anti-dilutive securities	110,365	129,491
Stock options.
Anti-dilutive securities
Anti-dilutive securities	3,905,845	3,282,849

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://www.xbrl.org/2003/role/presentationRef
-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

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Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration