Income Taxes (Reconciliation of U.S. Federal Statutory Tax Rate) (Details)	12 Months Ended
	May 31, 2022	May 31, 2021	May 31, 2020
Income Tax Disclosure [Abstract]
Federal statutory tax rate	21.00%	21.00%	21.00%
Increase/(decrease) resulting from:
State income taxes, net of federal tax benefit	4.60%	4.30%	4.00%
Tax-exempt municipal bond interest	(0.20%)	(0.30%)	(0.40%)
Stock option windfall benefit	(0.90%)	(1.20%)	(0.90%)
Tax credits	(1.10%)	(0.60%)	(0.50%)
Other items	0.30%	0.30%	0.40%
Effective income tax rate	23.70%	23.50%	23.60%