Commitments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	May 31, 2012 Month	May 31, 2011	May 31, 2010
Commitments and Contingencies Disclosure [Line Items]
Total Operating Lease Expense	$ 14.5	$ 15.5	$ 18.2
Lease payment due in 2013	15.4
Lease payment due in 2014	6.8
Lease payment due in 2015	4.4
Lease payment due in 2016	3.6
Lease payment due in 2017	3.4
Minimum purchases or payments during period	16.1	27.0	24.8
Total future minimum payments under the contract	6.1
Minimum purchases or payments in 2013	3.0
Minimum purchases or payments in 2014	0.8
Minimum purchases or payments in 2015	0.8
Minimum purchases or payments in 2016	0.8
Minimum purchases or payments in 2017	0.7
Capital lease obligation in 2012	2.4
Capital lease obligation, total	6.0
Capital lease obligation in 2013	0.4
Capital lease obligation in 2014	0.4
Capital lease obligation in 2015	0.4
Capital lease obligation in 2016	0.4
Capital lease obligation in 2017	0.4
Estimated time period for commissioning after restart of construction, in months	24
Incurred capital cost excluding capitalized interest	353.1
Capitalized interest cost relating to the project	68.7
Capital cost paid relating to the project	344.4
Minimum
Commitments and Contingencies Disclosure [Line Items]
Expected total capital cost of project excluding capitalized interest	365
Maximum
Commitments and Contingencies Disclosure [Line Items]
Expected total capital cost of project excluding capitalized interest	375
Non Cancelable Operating Leases
Commitments and Contingencies Disclosure [Line Items]
Total future lease payments due	$ 37.5