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
Nov. 30, 2018
May 31, 2018
Funds Held for Clients and Corporate Investments [Abstract]    
Due in one year or less, Amortized cost $ 193.3  
Due after one year through three years, Amortized cost 825.2  
Due after three years through five years, Amortized cost 1,035.7  
Due after five years, Amortized cost 945.4  
Available-for-sale securities, Amortized cost 2,999.6 $ 3,143.1
Due in one year or less, Fair value 192.9  
Due after one year through three years, Fair value 818.0  
Due after three years through five years, Fair value 1,014.7  
Due after five years, Fair value 928.7  
Available-for-sale securities, Fair value $ 2,954.3 $ 3,104.8