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Share-Based Awards and Options - Share Based Compensation Expense (Details) - USD ($)
$ in Millions
12 Months Ended
May. 31, 2015
May. 31, 2014
May. 31, 2013
May. 31, 2011
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Share-based compensation expense $ 21.1 $ 29.8 $ 18.4  
Income tax benefit $ (6.9) $ (7.1) $ (5.6)  
Stock Option Plan 2011        
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]        
Shares reserved for future issuance (in shares)       14,000,000