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Income Taxes - Summary of Valuation Allowance (Details) - USD ($) $ in Thousands | 12 Months Ended
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May 31, 2018 | May 31, 2017
Movement in Valuation Allowance [Roll Forward] |  |
Balance at beginning of period | $ (18,088) | $ (17,047)
Additions | (3,268) | (1,667)
Subtractions (1) | 10,054 | 626
Balance at end of period | $ (11,302) | $ (18,088)

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

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

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- Definition Movement in Valuation Allowance [Roll Forward] + References No definition available. + Details
Name: | ctas_MovementinValuationAllowanceRollForward
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://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 740 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=84230637&loc=d3e32537-109319 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant