Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements (Details)

v2.4.0.6
Fair Value Measurements (Details) (USD $)
In Millions, unless otherwise specified
May 31, 2012
May 31, 2011
Available-for-sale securities:    
Available-for-sale securities $ 3,059.0 $ 2,737.2
Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 3,059.0 2,737.2
Other securities 9.2 8.9
Liabilities:    
Other long-term liabilities 9.2 8.9
General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 1,333.9 1,050.5
Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 334.3 484.7
Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 501.0 373.7
Variable rate demand notes [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 889.8 828.3
Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities      
Other securities 9.2 8.9
Liabilities:    
Other long-term liabilities 9.2 8.9
Quoted prices in active markets (Level 1) [Member] | General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities      
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      
Quoted prices in active markets (Level 1) [Member] | Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities      
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      
Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 3,059.0 2,737.2
Other securities      
Liabilities:    
Other long-term liabilities      
Significant other observable Inputs (Level 2) [Member] | General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 1,333.9 1,050.5
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 334.3 484.7
Significant other observable Inputs (Level 2) [Member] | Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 501.0 373.7
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 889.8 828.3
Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities      
Other securities      
Liabilities:    
Other long-term liabilities      
Significant unobservable Inputs (Level 3) [Member] | General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities      
Significant unobservable Inputs (Level 3) [Member] | Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities      
Significant unobservable Inputs (Level 3) [Member] | Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities      
Significant unobservable Inputs (Level 3) [Member] | Variable rate demand notes [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities