Income Taxes (Reconciliation of U.S. Federal Statutory Tax Rate) (Details)	12 Months Ended
	May 31, 2021	May 31, 2020	May 31, 2019
Income Taxes [Abstract]
Federal statutory tax rate	21.00%	21.00%	21.00%
Increase/(decrease) resulting from:
State income taxes, net of federal tax benefit	4.30%	3.90%	4.30%
Tax-exempt municipal bond interest	(0.30%)	(0.40%)	(0.50%)
Stock option windfall benefit	(1.30%)	(0.80%)	(0.50%)
Other items, including adoption of ASC 606	(0.20%)	(0.10%)	0.10%
Effective income tax rate	23.50%	23.60%	24.40%