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Income Taxes - Summary of Valuation Allowance (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
May 31, 2020 | May 31, 2019
Movement in Valuation Allowance [Roll Forward] |  |
Balance at beginning of period | $ (7,308) | $ (11,302)
Subtractions | 897 | 3,994
Balance at end of period | $ (6,411) | $ (7,308)

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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://www.xbrl.org/2003/role/disclosureRef -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
Name: | us-gaap_DeferredTaxAssetsValuationAllowance
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant