Goodwill, Service Contracts and Other Assets - Changes in Carrying Amount of Service Contracts (Details) $ in Thousands	3 Months Ended
Aug. 31, 2017 USD ($)
Service contracts [Roll Forward]
Beginning balance	$ 586,988
Service contracts acquired	455
Service contracts amortization	(12,471)
Foreign currency translation	6,659
Ending balance	581,631
Uniform Rental and Facility Services
Service contracts [Roll Forward]
Beginning balance	529,923
Service contracts acquired	32
Service contracts amortization	(10,334)
Foreign currency translation	6,486
Ending balance	526,107
First Aid and Safety Services
Service contracts [Roll Forward]
Beginning balance	30,062
Service contracts acquired	411
Service contracts amortization	(958)
Foreign currency translation	173
Ending balance	29,688
All Other
Service contracts [Roll Forward]
Beginning balance	27,003
Service contracts acquired	12
Service contracts amortization	(1,179)
Foreign currency translation	0
Ending balance	$ 25,836

Amount of reduction of finite-lived intangible assets due to amortization during the period.

No definition available.

Name:	ctas_FiniteLivedIntangibleAssetsAmortization
Namespace Prefix:	ctas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of foreign currency translation gain (loss) which increases (decreases) assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsForeignCurrencyTranslationGainLoss
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount after amortization of assets, excluding financial assets and goodwill, lacking physical substance with a finite life.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16323-109275

+ Details

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

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

Name:	us-gaap_FiniteLivedIntangibleAssetsRollForward
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of increase in assets, excluding financial assets, lacking physical substance with a definite life, from an acquisition.

+ References

No definition available.

+ Details

Name:	us-gaap_FinitelivedIntangibleAssetsAcquired1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=ctas_UniformRentalandFacilityServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=ctas_FirstAidandSafetyServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=us-gaap_AllOtherSegmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: