Contingencies (Details) (USD $) In Thousands, unless otherwise specified	12 Months Ended	66 Months Ended
	Apr. 30, 2012	Apr. 30, 2012	Apr. 30, 2011
Contingencies [Abstract]
Accrued liability	$ 380	$ 380	$ 231
Number of lawsuits - "hot fuel" cases	4	45
Number of jurisdictions		27
Number of states		25
Annual stop loss limit	1,000
Letters of credit outstanding	15,000	15,000	12,000
Investments in escrow for insurance	223
Accrued expenses for estimated claims relating to self insurance	$ 23,701	$ 23,701	$ 22,129