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