Income Taxes - Uncertain tax positions (Details) - USD ($) $ in Thousands	12 Months Ended
	Apr. 30, 2020	Apr. 30, 2019
Income tax examinations
Reserve for uncertain tax positions	$ 0	$ 0
Liability for uncertain tax position	$ 0	$ 0
State | Minimum
Income tax examinations
Statute of limitation period	3 years
State | Maximum
Income tax examinations
Statute of limitation period	4 years