Income Taxes (Reconciliation of U.S. Federal Statutory Tax Rate) (Details)	12 Months Ended
May 31, 2017	May 31, 2016	May 31, 2015
Income Taxes [Abstract]
Federal statutory tax rate	35.00%	35.00%	35.00%
State income taxes, net of federal tax benefit	2.80%	3.00%	3.00%
Section 199 - Qualified production activities	(0.50%)	(2.40%)
Tax-exempt municipal bond interest	(1.30%)	(1.40%)	(1.50%)
Stock option windfall benefit	(1.50%)
Other items	(0.20%)	0.10%	(0.20%)
Effective income tax rate	34.30%	34.30%	36.30%

Effective Income Tax Rate Reconciliation Stock Option Windfall Benefit

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- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section 50
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Percentage of domestic federal statutory tax rate applicable to pretax income (loss).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
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-URI http://asc.fasb.org/extlink&oid=65882285&loc=d3e32687-109319

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-Publisher FASB
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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 deduction for qualified production activity.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 740
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB TOPIC 6.I)
-URI http://asc.fasb.org/extlink&oid=34349781&loc=d3e330036-122817

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- Definition

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 applicable to state and local income tax expense (benefit), net of federal tax expense (benefit).

+ References

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