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Description of Business, Basis of Presentation, and Significant Accounting Policies (Details) (USD $)
In Millions, unless otherwise specified
3 Months Ended 6 Months Ended 12 Months Ended
Nov. 30, 2012
Nov. 30, 2011
Nov. 30, 2012
Nov. 30, 2011
May 31, 2012
Description of business and significant accounting policies (Textual)          
Maximum individual workers' compensation claims liability     $ 1.0   $ 1.0
Workers' compensation insurance costs in current liabilities 5.7   5.7   6.2
Workers compensation insurance costs in long-term liabilities 14.2   14.2   19.0
Stock-based compensation costs recognized $ 5.1 $ 5.3 $ 11.5 $ 11.9