Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements (Details)

v3.5.0.2
Fair Value Measurements (Details) - USD ($)
$ in Millions
 12 Months Ended
May 31, 2016
May 31, 2015
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents   $ 103.1
Available-for-sale securities $ 4,141.9 3,595.6
Other 14.9 14.2
Other long-term liabilities 14.9 14.2
Commercial Paper [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents   15.0
Corporate Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 131.2  
General Obligation Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents   55.1
Available-for-sale securities 1,621.6 1,703.7
Pre-Refunded Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents   20.5
Available-for-sale securities 61.6 102.7
Revenue Municipal Bonds [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents   12.3
Available-for-sale securities 931.8 963.6
U.S. Government Agency Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 161.1  
Variable Rate Demand Notes [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities 1,234.6 825.6
Money Market Securities [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents   0.2
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   0.2
Other 14.9 14.2
Other long-term liabilities 14.9 14.2
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   0.2
Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Cash equivalents   102.9
Available-for-sale securities 4,141.9 3,595.6
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   15.0
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 131.2  
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]    
Cash equivalents   55.1
Available-for-sale securities 1,621.6 1,703.7
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]    
Cash equivalents   20.5
Available-for-sale securities 61.6 102.7
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]    
Cash equivalents   12.3
Available-for-sale securities 931.8 963.6
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 161.1  
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 1,234.6 825.6
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] | 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]    
Cash equivalents  
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]    
Cash equivalents  
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]    
Cash equivalents  
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
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  
Maximum [Member] | Significant Other Observable Inputs (Level 2) [Member] | Available-For-Sale Securities [Member] | Recurring [Member]    
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]    
Available-for-sale securities maturity 90 days