Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Millions	May 31, 2018	May 31, 2017
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 862.1
Available-for-sale securities	3,104.8	$ 4,613.2
Other	18.2	16.7
Other long-term liabilities	18.2	16.7
Commercial Paper [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	655.0
Time Deposits [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	200.0
Money Market Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	7.1
Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	321.4	210.8
General Obligation Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,221.7	1,442.3
Pre-Refunded Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	50.9	55.5
Revenue Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	806.4	940.9
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	396.1	325.8
Variable Rate Demand Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	308.3	1,637.9
Quoted Prices in Active Markets (Level 1) [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	7.1
Available-for-sale securities
Other	18.2	16.7
Other long-term liabilities	18.2	16.7
Quoted Prices in Active Markets (Level 1) [Member] | Commercial Paper [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Quoted Prices in Active Markets (Level 1) [Member] | Time Deposits [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Quoted Prices in Active Markets (Level 1) [Member] | Money Market Securities [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	7.1
Quoted Prices in Active Markets (Level 1) [Member] | Corporate Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Quoted Prices in Active Markets (Level 1) [Member] | General Obligation Municipal Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Quoted Prices in Active Markets (Level 1) [Member] | Pre-Refunded Municipal Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Quoted Prices in Active Markets (Level 1) [Member] | Revenue Municipal Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Quoted Prices in Active Markets (Level 1) [Member] | U.S. Government Agency Securities [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Quoted Prices in Active Markets (Level 1) [Member] | Variable Rate Demand Notes [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	855.0
Available-for-sale securities	3,104.8	4,613.2
Other
Other long-term liabilities
Significant Other Observable Inputs (Level 2) [Member] | Commercial Paper [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	655.0
Significant Other Observable Inputs (Level 2) [Member] | Time Deposits [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	200.0
Significant Other Observable Inputs (Level 2) [Member] | Money Market Securities [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Significant Other Observable Inputs (Level 2) [Member] | Corporate Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	321.4	210.8
Significant Other Observable Inputs (Level 2) [Member] | General Obligation Municipal Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,221.7	1,442.3
Significant Other Observable Inputs (Level 2) [Member] | Pre-Refunded Municipal Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	50.9	55.5
Significant Other Observable Inputs (Level 2) [Member] | Revenue Municipal Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	806.4	940.9
Significant Other Observable Inputs (Level 2) [Member] | U.S. Government Agency Securities [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	396.1	325.8
Significant Other Observable Inputs (Level 2) [Member] | Variable Rate Demand Notes [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	308.3	1,637.9
Significant Unobservable Inputs (Level 3) [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Available-for-sale securities
Other
Other long-term liabilities
Significant Unobservable Inputs (Level 3) [Member] | Commercial Paper [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Significant Unobservable Inputs (Level 3) [Member] | Time Deposits [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Significant Unobservable Inputs (Level 3) [Member] | Money Market Securities [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents
Significant Unobservable Inputs (Level 3) [Member] | Corporate Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Significant Unobservable Inputs (Level 3) [Member] | General Obligation Municipal Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Significant Unobservable Inputs (Level 3) [Member] | Pre-Refunded Municipal Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Significant Unobservable Inputs (Level 3) [Member] | Revenue Municipal Bonds [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Significant Unobservable Inputs (Level 3) [Member] | U.S. Government Agency Securities [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities
Significant Unobservable Inputs (Level 3) [Member] | Variable Rate Demand Notes [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities