| 
           Fair Value Measurements (Details) (USD $) 
        In Millions, unless otherwise specified  | 
        
           May 31, 2013 
         | 
        
           May 31, 2012 
         | 
      ||||
|---|---|---|---|---|---|---|
| Available-for-sale securities: | ||||||
| Available-for-sale securities | $ 3,691.4 | $ 3,059.0 | ||||
| 
           Fair Value, Measurements, Recurring [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 3,691.4 | 3,059.0 | ||||
| Other securities | 10.7 | 9.2 | ||||
| Liabilities: | ||||||
| Other long-term liabilities | 10.7 | 9.2 | ||||
| 
           Variable Rate Demand Obligation [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 1,276.7 | 889.8 | ||||
| 
           Variable Rate Demand Obligation [Member] | Fair Value, Measurements, Recurring [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 1,276.7 | 889.8 | ||||
| 
           Revenue municipal bonds [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 754.0 | 501.0 | ||||
| 
           Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 754.0 | 501.0 | ||||
| 
           Pre-refunded municipal bonds [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 203.9 | [1] | 334.3 | [1] | ||
| 
           Pre-refunded municipal bonds [Member] | Fair Value, Measurements, Recurring [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 203.9 | 334.3 | ||||
| 
           General obligation municipal bonds [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 1,456.8 | 1,333.9 | ||||
| 
           General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 1,456.8 | 1,333.9 | ||||
| 
           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 | 10.7 | 9.2 | ||||
| Liabilities: | ||||||
| Other long-term liabilities | 10.7 | 9.2 | ||||
| 
           Quoted prices in active markets (Level 1) [Member] | Variable Rate Demand Obligation [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] | 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] | General obligation municipal bonds [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 | 3,691.4 | 3,059.0 | ||||
| Other securities | 0 | 0 | ||||
| Liabilities: | ||||||
| Other long-term liabilities | 0 | 0 | ||||
| 
           Significant other observable Inputs (Level 2) [Member] | Variable Rate Demand Obligation [Member] | Fair Value, Measurements, Recurring [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 1,276.7 | 889.8 | ||||
| 
           Significant other observable Inputs (Level 2) [Member] | Revenue municipal bonds [Member] | Fair Value, Measurements, Recurring [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | 754.0 | 501.0 | ||||
| 
           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 | 203.9 | 334.3 | ||||
| 
           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 | 1,456.8 | 1,333.9 | ||||
| 
           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] | Variable Rate Demand Obligation [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] | 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] | General obligation municipal bonds [Member] | Fair Value, Measurements, Recurring [Member] 
         | 
        ||||||
| Available-for-sale securities: | ||||||
| Available-for-sale securities | $ 0 | $ 0 | ||||
          
  | 
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