Subsequent Event - Narrative (Details) - Service Line and Business Unit - Discontinued Operations, Held-for-sale or Disposed of by Sale - USD ($) $ / shares in Units, $ in Millions	12 Months Ended
May 31, 2017	May 31, 2016	May 31, 2015	Jul. 11, 2017
Subsequent Event [Line Items]
Revenue	$ 105.6	$ 109.7	$ 107.2
Diluted earnings per share (in dollars per share)	$ 0.07	$ 0.07	$ 0.07
Subsequent Event
Subsequent Event [Line Items]
Sale price				$ 130.0

Disposal Group, Including Discontinued Operation, Earnings Per Share, Diluted

No definition available.

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Namespace Prefix:	ctas_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of consideration received or receivable for the disposal of assets and liabilities, including discontinued operation.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount of revenue attributable to disposal group, including, but not limited to, discontinued operation.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 205
-SubTopic 20
-Section 50
-Paragraph 5B
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=64827811&loc=SL51721673-107760

+ Details

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- Definition

Detail information of subsequent event by type. User is expected to use existing line items from elsewhere in the taxonomy as the primary line items for this disclosure, which is further associated with dimension and member elements pertaining to a subsequent event.

+ References

No definition available.

+ Details

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- Details

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- Details

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- Details

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