Annual report pursuant to Section 13 and 15(d)

Income Taxes (Reconciliation of U.S. Federal Statutory Tax Rate) (Details)

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Income Taxes (Reconciliation of U.S. Federal Statutory Tax Rate) (Details)
12 Months Ended
May 31, 2016
May 31, 2015
May 31, 2014
Income Taxes [Abstract]      
Federal statutory tax rate 35.00% 35.00% 35.00%
State income taxes, net of federal tax benefit 3.00% 3.00% 3.30%
Section 199 - Qualified production activities (2.40%)    
Tax-exempt municipal bond interest (1.40%) (1.50%) (1.50%)
Other items 0.10% (0.20%) (0.30%)
Effective income tax rate 34.30% 36.30% 36.50%