Business, Basis of Presentation and Summary of Significant Accounting Policies - Insurance Liabilities (Details) - USD ($) $ in Millions	Jan. 31, 2018	Apr. 30, 2017
Other accrued expenses and current liabilities
Insurance Liabilities
Aggregate liabilities for medical self-insurance	$ 4.3	$ 3.4
General liability
Insurance Liabilities
Deductible amount	0.3
Workers' compensation
Insurance Liabilities
Deductible amount	0.5
Automobile
Insurance Liabilities
Deductible amount	1.0
General liability, workers' compensation and automobile
Insurance Liabilities
Reserve for insurance	15.8	15.9
General liability, workers' compensation and automobile | Prepaid expenses and other current assets
Insurance Liabilities
Insurance recovery receivable	6.2	$ 6.7
General liability, workers' compensation and automobile | Minimum
Insurance Liabilities
Primary layer of insurance coverage	0.5
Excess layer of insurance coverage	2.0
General liability, workers' compensation and automobile | Maximum
Insurance Liabilities
Primary layer of insurance coverage	2.0
Excess layer of insurance coverage	$ 100.0