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Fair Value Measurements - Narrative (Details) (USD $) | 9 Months Ended |
--- | --- | ---
Feb. 28, 2015 | May 31, 2014
Fair value on a recurring basis |  |
Marketable securities | $ 44,874,000 us-gaap_AvailableForSaleSecurities | $ 0 us-gaap_AvailableForSaleSecurities
Contractual maturities (less than) | 1 year |
Canadian treasury securities |  |
Fair value on a recurring basis |  |
Amortized cost basis of marketable securities | 44,900,000 us-gaap_AvailableForSaleSecuritiesAmortizedCost / us-gaap_InvestmentTypeAxis = ctas_CanadianTreasurySecuritiesMember |
Marketable securities |  | $ 0 us-gaap_AvailableForSaleSecurities / us-gaap_InvestmentTypeAxis = ctas_CanadianTreasurySecuritiesMember

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- Definition Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 25 -Paragraph 1 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=28360136&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=27724398&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=6871852&loc=d3e26610-111562 + Details
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- Definition This item represents the cost of debt and equity securities, which are categorized neither as held-to-maturity nor trading, net of adjustments including accretion, amortization, collection of cash, previous other-than-temporary impairments recognized in earnings (less any cumulative-effect adjustments recognized, as defined), and fair value hedge accounting adjustments, if any. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.2) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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