Fair value of financial assets and liabilities - Unobservable inputs of the warrant liability (Details)	Mar. 31, 2019 $ / shares	Jul. 31, 2018 $ / shares	Jul. 24, 2018 $ / shares
Initial public offering
Unobservable inputs of the warrant liability
Fair value of series seed preferred stock (in price per share)		$ 15.00	$ 15.00
Risk-free interest rate
Unobservable inputs of the warrant liability
Warrant liability inputs	2.81
Expected term
Unobservable inputs of the warrant liability
Warrant liability expected term (in years)	7 years 2 months 12 days
Expected volatility
Unobservable inputs of the warrant liability
Warrant liability inputs	64.4
Expected dividend yield
Unobservable inputs of the warrant liability
Warrant liability inputs	0
Fair value of series seed preferred stock
Unobservable inputs of the warrant liability
Warrant liability inputs	15.00