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Stock-Based Compensation - Narrative (Details) - USD ($)
$ / shares in Units, $ in Millions
12 Months Ended
May. 31, 2015
May. 31, 2014
May. 31, 2013
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Stock compensation expense resulting from the immediate vesting related to the shredding transaction   $ 12.4  
Weighted-average fair value of stock options granted (dollars per share) $ 22.64 $ 16.63 $ 9.60
Intrinsic value of options exercised $ 44.3 $ 19.8 $ 3.7
Proceeds from stock options exercised 40.2 41.9 14.8
Fair value of vested stock options 10.9 $ 17.7 13.2
Aggregate intrinsic value of outstanding options 269.1    
Aggregate intrinsic value of exercisable options $ 74.1    
Weighted-average remaining contractual term of stock options exercisable 4 years 7 months 6 days    
Unrecognized compensation cost related to unvested stock options and restricted stock $ 105.0    
Weighted-average period of time unrecognized compensation cost will be recognized 2 years 1 month 6 days    
Stock Compensation Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of shares authorized under plan (shares) 21,000,000    
Shares reserved for future issuance (shares) 9,281,640    
Stock Options      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of options vested as result of Shredding transaction   249,335  
Maximum term after grant options may be exercised 10 years    
Award expiration term 10 years    
Stock Options | Minimum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options vesting period 3 years    
Stock Options | Maximum      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Options vesting period 5 years    
Restricted Stock Awards      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Number of options vested as result of Shredding transaction   71,882  
Vesting period of awards 3 years    
Continuing Operations | Stock Compensation Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
Share-based compensation $ 44.9 $ 44.2 23.0
Continuing Operations | Income Taxes | Stock Compensation Plan      
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]      
The total income tax benefit recognized in the consolidated income statement for share-based compensation arrangements $ 16.7 $ 16.9 $ 8.4