Fair Value Of Financial Instruments (Assets And Liabilities Measured At Fair Value On Recurring Basis) (Detail) (USD $) In Thousands, unless otherwise specified	Jul. 31, 2012	Apr. 30, 2012	Jul. 31, 2012 Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] Fair Value, Measurements, Recurring [Member]	Jul. 31, 2011 Quoted Prices In Active Markets For Identical Assets (Level 1) [Member] Fair Value, Measurements, Recurring [Member]	Jul. 31, 2012 Significant Other Observable Inputs (Level 2) [Member] Fair Value, Measurements, Recurring [Member]	Jul. 31, 2011 Significant Other Observable Inputs (Level 2) [Member] Fair Value, Measurements, Recurring [Member]	Jul. 31, 2012 Significant Unobservable Inputs (Level 3) [Member] Fair Value, Measurements, Recurring [Member]	Jul. 31, 2011 Significant Unobservable Inputs (Level 3) [Member] Fair Value, Measurements, Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Restricted assets	$ 419	$ 424	$ 419	$ 329
Interest rate derivatives					3,626
Commodity derivatives						$ 47