Commitments - Additional Information (Detail) (USD $) In Millions, unless otherwise specified	9 Months Ended
	Feb. 28, 2013
Commitments and Contingencies Disclosure [Line Items]
Incurred capital cost excluding capitalized interest	$ 370.9
Capitalized interest cost relating to the project	98.1
Capital cost paid relating to the project	367.3
Minimum
Commitments and Contingencies Disclosure [Line Items]
Expected total capital cost of project excluding capitalized interest	370
Maximum
Commitments and Contingencies Disclosure [Line Items]
Expected total capital cost of project excluding capitalized interest	$ 375