Derivative Financial Instruments - Outstanding Derivative Contracts Entered into Hedge Forecasted Purchases and Future Cash Receipts (Detail) (USD $) In Thousands, unless otherwise specified	Apr. 30, 2013
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notional Amount	$ 160,074
Fair Value	4,620
Inventory Commodity [Member] | United States dollars [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notional Amount	151,234
Fair Value	4,378
Inventory Commodity [Member] | Minimum [Member] | United States dollars [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity	2013-05
Inventory Commodity [Member] | Maximum [Member] | United States dollars [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity	2014-10
Accounts receivable Commodity [Member] | British pounds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Notional Amount	8,840
Fair Value	$ 242
Accounts receivable Commodity [Member] | Minimum [Member] | British pounds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity	2013-05
Accounts receivable Commodity [Member] | Maximum [Member] | British pounds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Maturity	2013-10