Schedule of Summary of Derivative Hedges (Detail) (USD $) In Thousands, unless otherwise specified	6 Months Ended
	Nov. 30, 2013
Derivatives Not Designated As Hedging Instruments | Commodity Contracts
Derivative [Line Items]
Notional Amount	$ 41,720
Derivatives Not Designated As Hedging Instruments | Commodity Contracts | Minimum
Derivative [Line Items]
Maturity Date(s)	2013-12
Derivatives Not Designated As Hedging Instruments | Commodity Contracts | Maximum
Derivative [Line Items]
Maturity Date(s)	2015-01
Cash Flow Hedges | Commodity Contracts
Derivative [Line Items]
Notional Amount	16,940
Cash Flow Hedges | Commodity Contracts | Minimum
Derivative [Line Items]
Maturity Date(s)	2013-12
Cash Flow Hedges | Commodity Contracts | Maximum
Derivative [Line Items]
Maturity Date(s)	2014-10
Cash Flow Hedges | Interest Rate Contracts
Derivative [Line Items]
Notional Amount	$ 100,000
Cash Flow Hedges | Interest Rate Contracts | Minimum
Derivative [Line Items]
Maturity Date(s)	2013-12
Cash Flow Hedges | Interest Rate Contracts | Maximum
Derivative [Line Items]
Maturity Date(s)	2014-12