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Commitments and Contingencies (Tables)
3 Months Ended
Aug. 31, 2012
Commitments and Contingencies [Abstract]  
Lines of credit facility unused borrowing capacity
                 

Financial institution

  Amount available     Expiration date  

JP Morgan Chase Bank, N.A.

  $ 350 million       February 2013  

Bank of America, N.A.

  $ 250 million       February 2013  

PNC Bank, National Association

  $ 150 million       February 2013  

Wells Fargo Bank, National Association

  $ 150 million       February 2013