Net loss per share - Computation of diluted net loss per share attributable to common stockholders (Details) - shares	12 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2018
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	5,461,725	4,219,128	22,174,951
Options to purchase common stock
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	4,964,381	3,721,784	2,520,247
Convertible preferred stock
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of diluted weighted average shares outstanding			19,157,360
Warrants to purchase convertible preferred stock
Diluted net loss per share attributable to common stockholders
Antidilutive securities excluded from computation of diluted weighted average shares outstanding	497,344	497,344	497,344