Schedule of Summary of Derivative Hedges (Detail) (USD $) In Thousands, unless otherwise specified	3 Months Ended
	Aug. 31, 2013
Derivatives Not Designated As Hedging Instruments | Commodity Contracts
Derivative [Line Items]
Notional Amount	$ 28,830
Derivatives Not Designated As Hedging Instruments | Commodity Contracts | Minimum
Derivative [Line Items]
Maturity Date(s)	2013-09
Derivatives Not Designated As Hedging Instruments | Commodity Contracts | Maximum
Derivative [Line Items]
Maturity Date(s)	2014-12
Cash Flow Hedges | Commodity Contracts
Derivative [Line Items]
Notional Amount	37,040
Cash Flow Hedges | Commodity Contracts | Minimum
Derivative [Line Items]
Maturity Date(s)	2013-09
Cash Flow Hedges | Commodity Contracts | Maximum
Derivative [Line Items]
Maturity Date(s)	2014-12
Cash Flow Hedges | Interest Rate Contracts
Derivative [Line Items]
Notional Amount	$ 100,000
Maturity Date(s)	2014-12