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Income Taxes (Details 2)
12 Months Ended
May 31, 2011
May 31, 2010
May 31, 2009
Reconciliation of the U.S. federal statutory tax rate      
Federal statutory tax rate 35.00% 35.00% 35.00%
Increase/(decrease) resulting from:      
State income taxes, net of federal tax benefit 2.50% 2.30% 1.90%
Tax-exempt municipal bond interest (2.20%) (2.70%) (2.60%)
Other items (0.30%)    
Effective income tax rate 35.00% 34.60% 34.30%