| 
           Commitments and Contingencies (Details) (USD $) 
        In Millions, unless otherwise specified  | 
        12 Months Ended | 
|---|---|
| 
           May 31, 2013 
         | 
      |
| 
           JP Morgan Chase Bank, N.A. [Member] 
         | 
        |
| Lines of credit | |
| Amount available | $ 350 | 
| Expiration date | Feb. 28, 2014 | 
| 
           Bank of America, N.A. [Member] 
         | 
        |
| Lines of credit | |
| Amount available | 250 | 
| Expiration date | Feb. 28, 2014 | 
| 
           PNC Bank, National Association [Member] 
         | 
        |
| Lines of credit | |
| Amount available | 150 | 
| Expiration date | Feb. 28, 2014 | 
| 
           Wells Fargo Bank, National Association [Member] 
         | 
        |
| Lines of credit | |
| Amount available | $ 150 | 
| Expiration date | Feb. 28, 2014 | 
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- Details 
          
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- Definition 
          Date the credit facility terminates, in CCYY-MM-DD format. Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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- Definition 
          Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding). Reference 1: http://www.xbrl.org/2003/role/presentationRef 
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