Income Taxes (Reconciliation of U.S. Federal Statutory Tax Rate) (Details)	12 Months Ended
May 31, 2018	May 31, 2017	May 31, 2016
Income Taxes [Abstract]
Federal statutory tax rate	29.20%	35.00%	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	3.00%	2.80%	3.00%
Section 199 - Qualified production activities	(0.30%)	(0.50%)	(2.40%)
Tax-exempt municipal bond interest	(1.10%)	(1.30%)	(1.40%)
Stock option windfall benefit	(0.80%)	(1.50%)
Other items		(0.20%)	0.10%
Effective income tax rate	28.00%	34.30%	34.30%

Effective Income Tax Rate Reconciliation Stock Option Windfall Benefit

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Percentage of current income tax expense (benefit) and deferred income tax expense (benefit) pertaining to continuing operations.

+ References

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-Publisher FASB
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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

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Percentage of the difference between reported income tax expense (benefit) and expected income tax expense (benefit) computed by applying the domestic federal statutory income tax rates to pretax income (loss) from continuing operations attributable to changes in the income tax rates.

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