Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Jan. 31, 2013	Apr. 30, 2012
Other Current Assets [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	$ 11,460	$ 10,171
Other Current Assets [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	10,276	3,602
Other Current Assets [Member] | Commodity contracts [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	1,184	6,569
Other Current Assets [Member] | Commodity contracts [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	9,157	3,166
Other Current Assets [Member] | Foreign currency exchange contracts [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	1,119	436
Other Current Liabilities [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	4,394	24,123
Other Current Liabilities [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	3,471	4,613
Other Current Liabilities [Member] | Commodity contracts [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	923	19,510
Other Current Liabilities [Member] | Commodity contracts [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	3,364	3,631
Other Current Liabilities [Member] | Foreign currency exchange contracts [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	$ 107	$ 982