Annual report pursuant to Section 13 and 15(d)

Funds Held for Clients and Corporate Investments (Amortized Cost and Fair Value of Available-for-sale Securities by Contractual Maturity) (Details)

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Funds Held for Clients and Corporate Investments (Amortized Cost and Fair Value of Available-for-sale Securities by Contractual Maturity) (Details) - USD ($)
$ in Millions
May 31, 2016
May 31, 2015
Funds Held for Clients and Corporate Investments [Abstract]    
Due in one year or less, Amortized cost $ 359.6  
Due after one year through three years, Amortized cost 742.7  
Due after three years through five years, Amortized cost 918.5  
Due after five years, Amortized cost 2,073.5  
Available-for-sale securities, Amortized cost 4,094.3 $ 3,582.0
Due in one year or less, Fair value 360.7  
Due after one year through three years, Fair value 749.4  
Due after three years through five years, Fair value 935.7  
Due after five years, Fair value 2,096.1  
Available-for-sale securities, Fair value $ 4,141.9 $ 3,595.6