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Stock warrants - Summary of all outstanding and exercisable warrants (Details)
3 Months Ended
Mar. 31, 2020
$ / shares
shares
Exercise Price 1.38 [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Exercise Price | $ / shares $ 1.38
Warrants Outstanding 17,999,999
Warrants Exercisable 17,999,999
Weighted Average Remaining Contractual Life (in years) 3 years 6 months 11 days
Exercise Price 18.20 [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Exercise Price | $ / shares $ 18.20
Warrants Outstanding 714
Warrants Exercisable 714
Weighted Average Remaining Contractual Life (in years) 2 years 8 months 27 days
Exercise Price 50.05 [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Exercise Price | $ / shares $ 50.05
Warrants Outstanding 714,286
Warrants Exercisable 714,286
Weighted Average Remaining Contractual Life (in years) 7 months 21 days
Exercise price 3.24 [Member]  
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]  
Exercise Price | $ / shares $ 3.24
Warrants Outstanding 18,714,999
Warrants Exercisable 18,714,999
Weighted Average Remaining Contractual Life (in years) 3 years 5 months 1 day