Fair Value Measurements (Financial Assets and Liabilities Measured at Fair Value on Recurring Basis) (Details) - USD ($) $ in Millions	9 Months Ended
	Feb. 28, 2017	May 31, 2016
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	$ 18.7
Available-for-sale securities	4,460.3	$ 4,141.9
Other	16.1	14.9
Other long-term liabilities	16.1	14.9
Corporate Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	189.7	131.2
General Obligation Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	3.7
Available-for-sale securities	1,464.9	1,621.6
Pre-Refunded Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	63.6	61.6
Revenue Municipal Bonds [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	15.0
Available-for-sale securities	949.0	931.8
U.S. Government Agency Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	290.5	161.1
Variable Rate Demand Notes [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities	1,502.6	1,234.6
Quoted Prices in Active Markets (Level 1) [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Other	16.1	14.9
Other long-term liabilities	16.1	14.9
Significant Other Observable Inputs (Level 2) [Member] | Recurring [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Cash equivalents	18.7
Available-for-sale securities	4,460.3	4,141.9
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	189.7	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	3.7
Available-for-sale securities	1,464.9	1,621.6
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	63.6	61.6
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	15.0
Available-for-sale securities	949.0	931.8
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	290.5	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	1,502.6	1,234.6
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] | 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]
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
Maximum [Member] | Significant Other Observable Inputs (Level 2) [Member] | Available-For-Sale Securities [Member]
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Line Items]
Available-for-sale securities maturity	90 days