Stockholders' equity - Grant date fair value of warrants, valuation assumptions (Details)	Sep. 12, 2019 $ / shares
Measurement Input, Risk Free Interest Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants outstanding, measurement input	0.0167
Measurement Input, Expected Term
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants outstanding, measurement input	3
Measurement Input, Price Volatility
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants outstanding, measurement input	0.6510
Measurement Input, Expected Dividend Rate
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants outstanding, measurement input	0.0000
Measurement Input, Share Price
Fair Value Measurement Inputs and Valuation Techniques [Line Items]
Warrants outstanding, measurement input	6.72