Fair value of financial assets and liabilities - Roll forward of the warrant liability (Details) - Warrants to purchase convertible preferred stock $ in Thousands	12 Months Ended
	Mar. 31, 2019 USD ($)
Roll forward of the warrant liability
Balance at beginning of the period	$ 1,642
Change in fair value	5,452
Conversion of convertible preferred stock warrant into common stock warrant	$ (7,094)