Income Taxes (Narrative) (Details) - State [Member] - USD ($) $ in Millions	12 Months Ended
	Apr. 30, 2017	Apr. 30, 2016
Tax Credit Carryforward [Line Items]
Gross amount of state tax credit carryforwards	$ 6.1	$ 5.8
Net deferred tax assets related to tax credit carryforwards	1.6	1.8
Other Liabilities [Member]
Tax Credit Carryforward [Line Items]
Net deferred tax assets related to tax credit carryforwards	1.5	$ 1.8
Investment Tax Credit Carryforward [Member]
Tax Credit Carryforward [Line Items]
Decrease in valuation allowance	$ (0.6)