Stock-Based Compensation, Director Stock Awards (Details) - Director Stock Plan [Member] - Class A Common Stock [Member] - Non-Employee Directors [Member] - shares	12 Months Ended
	Apr. 30, 2018	Apr. 30, 2017	Apr. 30, 2016
Deferred Compensation Arrangement with Individual, Share-based Payments [Line Items]
Value of annual award as percentage of annual director retainer fee based on stock price on date of grant	100.00%
Shares awarded under the plan (in shares)	19,900	20,243	19,559