Derivative Financial Instruments (Details) (USD $) In Thousands, unless otherwise specified	Oct. 31, 2012	Apr. 30, 2012
Other Current Assets [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	$ 17,755	$ 10,171
Other Current Assets [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	14,842	3,602
Other Current Assets [Member] | Commodity contracts - net [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	2,913	6,569
Other Current Assets [Member] | Commodity contracts - net [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	13,557	3,166
Other Current Assets [Member] | Foreign currency exchange contracts - net [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Assets	1,285	436
Other Current Liabilities [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	15,499	24,123
Other Current Liabilities [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	7,089	4,613
Other Current Liabilities [Member] | Commodity contracts - net [Member] | Designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	8,410	19,510
Other Current Liabilities [Member] | Commodity contracts - net [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	6,634	3,631
Other Current Liabilities [Member] | Foreign currency exchange contracts - net [Member] | Not designated as hedging instruments [Member]
Fair value of derivative instruments [Line Items]
Derivatives Instruments, Liabilities	$ 455	$ 982