Retirement Plan (Details) - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Retirement Plan
Employee contributions matched by employer (as a percent)	50.00%
Employee compensation eligible for employer match of employee contributions (as a percent)	4.00%
Employer contributions to defined contribution retirement plan	$ 4.3	$ 3.7	$ 1.7