Ordinary Shares - Additional Information (Details)	12 Months Ended
	Apr. 30, 2020 USD ($) shares	Apr. 30, 2020 € / shares	Apr. 30, 2019 shares	Oct. 31, 2018 shares	Sep. 30, 2018 € / shares shares
Class of Stock [Line Items]
Ordinary shares, shares authorized (in shares) | shares	165,000,000			72,000,000	72,000,000
Par value of shares issued ( in € / shares)					€ 0.001
Ordinary shares, voting rights	one vote per ordinary share
Dividends declared | $	$ 0
Number of shares unvested (in shares) | shares	0		0
Minimum
Class of Stock [Line Items]
Par value of shares issued ( in € / shares)		€ 0.001
Maximum
Class of Stock [Line Items]
Par value of shares issued ( in € / shares)		€ 0.01