Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details)

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Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($)
$ in Millions
Feb. 28, 2019
May 31, 2018
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted cash equivalents $ 1,878.4 $ 862.1
Available-for-sale securities 2,557.3 3,104.8
Other 29.1 18.2
Other long-term liabilities 29.1 18.2
Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted cash equivalents 65.0 655.0
Time Deposits [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted cash equivalents   200.0
Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 408.1 321.4
General Obligation Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 871.1 1,221.7
Pre-Refunded Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 48.0 50.9
Revenue Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 562.9 806.4
U.S. Government Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted cash equivalents 1,690.1  
Available-for-sale securities 485.3 396.1
Money Market Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted cash equivalents 123.3 7.1
Variable Rate Demand Notes [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 181.9 308.3
Quoted Prices in Active Markets (Level 1) [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted cash equivalents 123.3 7.1
Available-for-sale securities
Other 29.1 18.2
Other long-term liabilities 29.1 18.2
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]    
Restricted 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]    
Restricted cash equivalents  
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]    
Restricted cash equivalents  
Available-for-sale securities
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]    
Restricted cash equivalents 123.3 7.1
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]    
Restricted cash equivalents 1,755.1 855.0
Available-for-sale securities 2,557.3 3,104.8
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]    
Restricted cash equivalents 65.0 655.0
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]    
Restricted cash equivalents   200.0
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 408.1 321.4
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 871.1 1,221.7
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 48.0 50.9
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 562.9 806.4
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]    
Restricted cash equivalents 1,690.1  
Available-for-sale securities 485.3 396.1
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]    
Restricted cash equivalents
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 181.9 308.3
Significant Unobservable Inputs (Level 3) [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Restricted 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]    
Restricted 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]    
Restricted 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]    
Restricted cash equivalents  
Available-for-sale securities
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]    
Restricted cash equivalents
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