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, 2019
May 31, 2018
May 31, 2017
Income Taxes [Abstract]      
Federal statutory tax rate 21.00% 29.20% 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 4.30% 3.00% 2.80%
Section 199 - Qualified production activities   (0.30%) (0.50%)
Tax-exempt municipal bond interest (0.50%) (1.10%) (1.30%)
Stock option windfall benefit (0.50%) (0.80%) (1.50%)
Other items, including adoption of ASC 606 0.10% (4.50%) (0.10%)
Effective income tax rate 24.40% 23.50% 34.40%