Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Details)

v2.4.0.6
Fair Value Measurements (Details) (USD $)
In Millions, unless otherwise specified
Aug. 31, 2012
May 31, 2012
Available-for-sale securities:    
Available-for-sale securities $ 3,407.9 $ 3,059.0
Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 3,407.9 3,059.0
Other securities 10.2 9.2
Liabilities:    
Other long-term liabilities 10.2 9.2
General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 1,411.3 1,333.9
Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 242.0 334.3
Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 601.2 501.0
Variable rate demand notes [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 1,153.4 889.8
Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities      
Other securities 10.2 9.2
Liabilities:    
Other long-term liabilities 10.2 9.2
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 3,407.9 3,059.0
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 1,411.3 1,333.9
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 242.0 334.3
Significant other observable Inputs (Level 2) [Member] | Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 601.2 501.0
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 1,153.4 889.8
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