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%