Stockholders' equity - Warrants (Details) - Warrant - $ / shares	Mar. 31, 2020	Dec. 31, 2019
Number of options
Outstanding (in shares)	1,662,304	1,662,304
Exercisable (in shares)	1,662,304
Weighted average exercise price
Outstanding (in dollars per share)	$ 3.25	$ 3.25
Exercisable (in dollars per share)	$ 3.25

Exercise price per share or per unit of warrants or rights outstanding.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 235
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.4-08.(i)(4))
-URI http://asc.fasb.org/extlink&oid=26873400&loc=d3e23780-122690

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Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant