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Income Taxes (Summary of Valuation Allowance) (Details) (USD $) In Thousands, unless otherwise specified | 12 Months Ended
--- | ---
May 31, 2014 | May 31, 2013
Movement in Valuation Allowance [Roll Forward] |  |
Balance at beginning of period | $ (12,789) | $ (9,054)
Additions | (1,701) | (7,391)
Subtractions | 1,132 | 3,656
Balance at end of period | $ (13,358) | $ (12,789)

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- Definition Deferred Tax Assets, Valuation Allowance, Additions + References No definition available. + Details
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Namespace Prefix: | ctas_
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Balance Type: | credit
Period Type: | duration

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- Definition Deferred Tax Assets, Valuation Allowance, Subtractions + References No definition available. + Details
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Namespace Prefix: | ctas_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Details
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Namespace Prefix: | ctas_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Amount of deferred tax assets for which it is more likely than not that a tax benefit will not be realized. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=6907707&loc=d3e32537-109319 + Details
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