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
Feb. 28, 2013
May 31, 2012
Available-for-sale securities:    
Available-for-sale securities $ 4,873.7 $ 3,059.0
Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 4,873.7 3,059.0
Other securities 10.3 9.2
Liabilities:    
Other long-term liabilities 10.3 9.2
General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 1,380.2 1,333.9
Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 211.4 334.3
Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 682.0 501.0
Variable rate demand notes [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 2,600.1 889.8
Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 0 0
Other securities 10.3 9.2
Liabilities:    
Other long-term liabilities 10.3 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 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]
   
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]
   
Available-for-sale securities:    
Available-for-sale securities 4,873.7 3,059.0
Other securities 0 0
Liabilities:    
Other long-term liabilities 0 0
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 1,380.2 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 211.4 334.3
Significant other observable Inputs (Level 2) [Member] | Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 682.0 501.0
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 2,600.1 889.8
Significant unobservable Inputs (Level 3) [Member] | Fair Value, Measurements, Recurring [Member]
   
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] | General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
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]
   
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