Quarterly report pursuant to Section 13 or 15(d)

Description of Business, Basis of Presentation, and Significant Accounting Policies (Details)

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Description of Business, Basis of Presentation, and Significant Accounting Policies (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 12 Months Ended
Aug. 31, 2012
Aug. 31, 2011
May 31, 2012
Description of business and significant accounting policies (Textual) [Abstract]      
Maximum individual workers' compensation claims liability $ 1.0   $ 1.0
Workers' compensation insurance costs in current liabilities 6.7   6.2
Workers compensation insurance costs in long-term liabilities 14.6   19.0
Stock-based compensation costs recognized $ 6.4 $ 6.6