| 
           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 |