Quarterly report pursuant to Section 13 or 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
Feb. 28, 2019
May 31, 2018
Funds Held for Clients and Corporate Investments [Abstract]    
Due in one year or less, Amortized cost $ 154.6  
Due after one year through three years, Amortized cost 810.0  
Due after three years through five years, Amortized cost 989.8  
Due after five years, Amortized cost 612.7  
Available-for-sale securities, Amortized cost 2,567.1 $ 3,143.1
Due in one year or less, Fair value 154.5  
Due after one year through three years, Fair value 808.7  
Due after three years through five years, Fair value 984.9  
Due after five years, Fair value 609.2  
Available-for-sale securities, Fair value $ 2,557.3 $ 3,104.8