Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Aug. 31, 2013	May 31, 2013
Available-for-sale securities:
Available-for-sale securities	$ 3,308.5	$ 3,691.4
Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	11.4
Available-for-sale securities:
Available-for-sale securities	3,308.5	3,691.4
Other securities	11.6	10.7
Liabilities:
Other long-term liabilities	11.6	10.7
Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	3.0
Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0.1
General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	6.3
Available-for-sale securities:
Available-for-sale securities	1,600.4	1,456.8
Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities:
Available-for-sale securities	131.4	203.9
Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	2.0
Available-for-sale securities:
Available-for-sale securities	830.8	754.0
Variable rate demand notes [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities:
Available-for-sale securities	745.9	1,276.7
Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0.1
Available-for-sale securities:
Available-for-sale securities	0	0
Other securities	11.6	10.7
Liabilities:
Other long-term liabilities	11.6	10.7
Quoted prices in active markets (Level 1) [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Quoted prices in active markets (Level 1) [Member] | Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0.1
Quoted prices in active markets (Level 1) [Member] | General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Available-for-sale securities:
Available-for-sale securities	0	0
Quoted prices in active markets (Level 1) [Member] | Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Quoted prices in active markets (Level 1) [Member] | Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Available-for-sale securities:
Available-for-sale securities	0	0
Quoted prices in active markets (Level 1) [Member] | Variable rate demand notes [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	11.3
Available-for-sale securities:
Available-for-sale securities	3,308.5	3,691.4
Other securities	0	0
Liabilities:
Other long-term liabilities	0	0
Significant other observable Inputs (Level 2) [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	3.0
Significant other observable Inputs (Level 2) [Member] | Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Significant other observable Inputs (Level 2) [Member] | General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	6.3
Available-for-sale securities:
Available-for-sale securities	1,600.4	1,456.8
Significant other observable Inputs (Level 2) [Member] | Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities:
Available-for-sale securities	131.4	203.9
Significant other observable Inputs (Level 2) [Member] | Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	2.0
Available-for-sale securities:
Available-for-sale securities	830.8	754.0
Significant other observable Inputs (Level 2) [Member] | Variable rate demand notes [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities:
Available-for-sale securities	745.9	1,276.7
Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Available-for-sale securities:
Available-for-sale securities	0	0
Other securities	0	0
Liabilities:
Other long-term liabilities	0	0
Significant unobservable Inputs (Level 3) [Member] | Commercial Paper [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Significant unobservable Inputs (Level 3) [Member] | Money Market Funds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Significant unobservable Inputs (Level 3) [Member] | General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Available-for-sale securities:
Available-for-sale securities	0	0
Significant unobservable Inputs (Level 3) [Member] | Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities:
Available-for-sale securities	0	0
Significant unobservable Inputs (Level 3) [Member] | Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
Cash equivalents:
Cash equivalents	0
Available-for-sale securities:
Available-for-sale securities	0	0
Significant unobservable Inputs (Level 3) [Member] | Variable rate demand notes [Member] | Fair Value, Measurements, Recurring [Member]
Available-for-sale securities:
Available-for-sale securities	$ 0	$ 0