Annual report pursuant to Section 13 and 15(d)

Funds Held for Clients and Corporate Investments (Details 2)

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Funds Held for Clients and Corporate Investments (Details 2) (USD $)
In Millions, unless otherwise specified
 12 Months Ended
May 31, 2012
Security
May 31, 2011
Security
May 31, 2010
Realized gains and losses from the sale of available-for-sale securities      
Gross realized gains $ 1.0 $ 1.3 $ 3.2
Gross realized losses         
Net realized gains 1.0 1.3 3.2
Maturity Date:      
Due in one year or less, Amortized cost 323.1    
Due in one year or less, Fair value 325.8    
Due after one year through three years, Amortized cost 619.6    
Due after one year through three years, Fair value 641.0    
Due after three years through five years, Amortized cost 630.2    
Due after three years through five years, Fair value 655.7    
Due after five years, Amortized cost 1,426.6    
Due after five years, Fair value 1,436.5    
Total, Amortized cost 2,999.5    
Total, Fair value 3,059.0 2,737.2  
Funds Held for Clients and Corporate Investments (Textual) [Abstract]      
Maturity period of municipal bonds included in cash equivalents less than 90 days    
Separately designated demand deposit account 13.7 16.7  
Net unrealized gain on available-for-sale securities 59.5 59.3  
Number of available-for-sale securities in an unrealized loss position 35 15  
Gross unrealized losses $ 0.3 $ 0.1  
Minimum maturity period for VRDNs, primarily categorized as due after five years (in years) 20 years    
Maximum maturity period for VRDNs, primarily categorized as due after five years (in years) 30 years