Quarterly report pursuant to Section 13 or 15(d)

Fair Value Measurements (Details)

 v2.3.0.11
Fair Value Measurements (Details) (USD $)
In Millions
Aug. 31, 2011
May 31, 2011
Available-for-sale securities:    
Available-for-sale securities $ 2,294.9 $ 2,737.2
Quoted prices in active markets (Level 1) [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Other securities 8.9 8.9
Liabilities:    
Other long-term liabilities 8.9 8.9
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 1,077.8 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 451.3 484.7
Significant other observable Inputs (Level 2) [Member] | Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 399.2 373.7
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 366.6 828.3
Significant other observable Inputs (Level 2) [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 2,294.9 2,732.2
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
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
General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 1,077.8 1,050.5
Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 451.3 484.7
Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 399.2 373.7
Variable rate demand notes [Member] | Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 366.6 828.3
Fair Value, Measurements, Recurring [Member]
   
Available-for-sale securities:    
Available-for-sale securities 2,294.9 2,732.2
Other securities 8.9 8.9
Liabilities:    
Other long-term liabilities $ 8.9 $ 8.9