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Stockholders' equity - Recent Issuances (Details) - March 2019 Public Offering [Member]
$ in Millions
1 Months Ended
Mar. 31, 2019
USD ($)
shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Proceeds from public offering | $ $ 6.6
Common Stock  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Issued (in shares) | shares 3,345,282