Goodwill, Service Contracts and Other Assets - Changes in Carrying Amount of Goodwill (Details) (USD $) In Thousands, unless otherwise specified	9 Months Ended
Feb. 28, 2015
Goodwill [Roll Forward]
Balance as of June 1, 2014	$ 1,267,411us-gaap_Goodwill
Goodwill acquired	7,386us-gaap_GoodwillAcquiredDuringPeriod
Goodwill divested in the Storage transactions	(75,660)
Foreign currency translation	(4,748)us-gaap_GoodwillTranslationAdjustments
Balance as of February 28, 2015	1,194,389us-gaap_Goodwill
Rental Uniforms & Ancillary Products
Goodwill [Roll Forward]
Balance as of June 1, 2014	943,516
Goodwill acquired	70
Goodwill divested in the Storage transactions	0
Foreign currency translation	(2,204)
Balance as of February 28, 2015	941,382
Uniform Direct Sales
Goodwill [Roll Forward]
Balance as of June 1, 2014	23,905
Goodwill acquired	0
Goodwill divested in the Storage transactions	0
Foreign currency translation	(125)
Balance as of February 28, 2015	23,780
First Aid, Safety & Fire Protection
Goodwill [Roll Forward]
Balance as of June 1, 2014	221,911
Goodwill acquired	7,316
Goodwill divested in the Storage transactions	0
Foreign currency translation	0
Balance as of February 28, 2015	229,227
Corporate
Goodwill [Roll Forward]
Balance as of June 1, 2014	78,079
Goodwill acquired	0
Goodwill divested in the Storage transactions	(75,660)
Foreign currency translation	(2,419)
Balance as of February 28, 2015	$ 0

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 45

-Paragraph 1

-URI http://asc.fasb.org/extlink&oid=6388280&loc=d3e13770-109266

+ Details

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

- Definition

Amount of increase in asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized resulting from a business combination.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

+ Details

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

- Details

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

- Definition

Amount of increase (decrease) from foreign currency translation adjustments of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (f)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

+ Details

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

- Definition

Amount of divestiture of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 350

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (d)

-URI http://asc.fasb.org/extlink&oid=35741047&loc=d3e13816-109267

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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