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%