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, 2020
May 31, 2019
May 31, 2018
May 31, 2017
Income Taxes [Abstract]        
Federal statutory tax rate 21.00% 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 3.90% 4.30% 3.00%  
Section 199 - Qualified production activities     (0.30%)  
Tax-exempt municipal bond interest (0.40%) (0.50%) (1.10%)  
Stock option windfall benefit (0.80%) (0.50%) (0.80%)  
Other items, including adoption of ASC 606 (0.10%) 0.10% (4.50%)  
Effective income tax rate 23.60% 24.40% 23.50%