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, 2018
May 31, 2017
Funds Held for Clients and Corporate Investments [Abstract]    
Due in one year or less, Amortized cost $ 317.9  
Due after one year through three years, Amortized cost 820.5  
Due after three years through five years, Amortized cost 998.1  
Due after five years, Amortized cost 1,654.0  
Available-for-sale securities, Amortized cost 3,790.5 $ 4,581.2
Due in one year or less, Fair value 317.9  
Due after one year through three years, Fair value 819.9  
Due after three years through five years, Fair value 990.7  
Due after five years, Fair value 1,627.1  
Available-for-sale securities, Fair value $ 3,755.6 $ 4,613.2