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, 2018
May 31, 2017
May 31, 2016
Income Taxes [Abstract]      
Federal statutory tax rate 29.20% 35.00% 35.00%
Increase/(decrease) resulting from:      
Statutory tax rate reduction resulting from the Tax Act (2.00%)    
State income taxes, net of federal tax benefit 3.00% 2.80% 3.00%
Section 199 - Qualified production activities (0.30%) (0.50%) (2.40%)
Tax-exempt municipal bond interest (1.10%) (1.30%) (1.40%)
Stock option windfall benefit (0.80%) (1.50%)  
Other items   (0.20%) 0.10%
Effective income tax rate 28.00% 34.30% 34.30%