Acquisitions and Deconsolidation (Aggregate Purchase Price for Businesses Acquired) (Details) (USD $) In Thousands, unless otherwise specified	May 31, 2014	May 31, 2013	May 31, 2012
Business Acquisition [Line Items]
Net goodwill recognized	$ 1,267,411	$ 1,517,560	$ 1,485,375
Series of business acquisitions
Business Acquisition [Line Items]
Fair value of tangible assets acquired	11,415	7,212
Fair value of service contracts acquired	6,343	34,858
Fair value of other intangibles acquired	924	2,049
Net goodwill recognized	13,865	32,133
Total fair value of assets acquired	32,547	76,252
Fair value of liabilities (settled) assumed and incurred	(894)	6,882
Total cash paid for acquisitions	$ 33,441	$ 69,370

Business Acquisition, Purchase Price Allocation, Intangible Assets Other than Goodwill

No definition available.

Name:	ctas_BusinessAcquisitionPurchasePriceAllocationAmortizableIntangibleAssetsOtherThanServiceContracts
Namespace Prefix:	ctas_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Business Acquisition, Purchase Price Allocations, Liabilities Assumed And Incurred

+ References

No definition available.

+ Details

Name:	ctas_BusinessAcquisitionPurchasePriceAllocationsLiabilitiesAssumedAndIncurred
Namespace Prefix:	ctas_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Assets, Including Goodwill

+ References

No definition available.

+ Details

Name:	ctas_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedAssetsIncludingGoodwill
Namespace Prefix:	ctas_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Business Combination, Recognized Identifiable Assets Acquired and Liabilities Assumed, Finite-Lived Intangibles, Service Contracts

+ References

No definition available.

+ Details

Name:	ctas_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedFiniteLivedIntangiblesServiceContracts
Namespace Prefix:	ctas_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

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

- Definition

The amount of property, plant, and equipment recognized as of the acquisition date.

+ References

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 20

-Section 50

-Paragraph 1

-Subparagraph (c)

-URI http://asc.fasb.org/extlink&oid=6910749&loc=d3e4845-128472

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

-Publisher FASB

-Name Accounting Standards Codification

-Topic 805

-SubTopic 10

-Section 55

-Paragraph 37

-URI http://asc.fasb.org/extlink&oid=25498275&loc=d3e2207-128464

+ Details

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

- Definition

Amount recognized for assets, including goodwill, in excess of (less than) the aggregate liabilities assumed.

+ References

+ Details

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

- Definition

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.

+ References

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=14024403&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