Net loss per share - Antidilutive securities (Details) - shares	3 Months Ended
Mar. 31, 2020	Mar. 31, 2019
Anti-dilutive securities
Anti-dilutive shares	2,903,977	19,999,531
Series A Preferred Stock
Anti-dilutive securities
Anti-dilutive shares		1,604,574
Series B Preferred Stock
Anti-dilutive securities
Anti-dilutive shares		5,730,612
Series C Preferred Stock
Anti-dilutive securities
Anti-dilutive shares		8,074,447
Series D Preferred Stock
Anti-dilutive securities
Anti-dilutive shares		2,992,260
Options
Anti-dilutive securities
Anti-dilutive shares	2,903,977	1,597,638

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

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