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Investments - Narrative (Details) (USD $)
3 Months Ended 6 Months Ended
Nov. 30, 2014
Nov. 30, 2013
Nov. 30, 2014
Nov. 30, 2013
May 31, 2014
Apr. 30, 2014
Schedule of Equity Method Investments [Line Items]            
Investments $ 456,864,000us-gaap_Investments   $ 456,864,000us-gaap_Investments   $ 458,357,000us-gaap_Investments  
Cash surrender value of insurance policies 97,500,000us-gaap_CashSurrenderValueOfLifeInsurance   97,500,000us-gaap_CashSurrenderValueOfLifeInsurance   86,500,000us-gaap_CashSurrenderValueOfLifeInsurance  
Cost method investments 2,000,000us-gaap_CostMethodInvestments   2,000,000us-gaap_CostMethodInvestments   800,000us-gaap_CostMethodInvestments  
Cost-method investments, losses due to impairment 0us-gaap_ImpairmentOfInvestments 0us-gaap_ImpairmentOfInvestments 0us-gaap_ImpairmentOfInvestments 0us-gaap_ImpairmentOfInvestments    
Gain on deconsolidation of Shredding business 0us-gaap_DeconsolidationGainOrLossAmount 0us-gaap_DeconsolidationGainOrLossAmount 6,619,000us-gaap_DeconsolidationGainOrLossAmount 0us-gaap_DeconsolidationGainOrLossAmount    
Dividends received on equity method investment     5,247,000us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital 0us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital    
Shred-it Partnership            
Schedule of Equity Method Investments [Line Items]            
Equity method investments 341,000,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_ShreditPartnershipMember
  341,000,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_ShreditPartnershipMember
  341,400,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_ShreditPartnershipMember
 
Percentage of newly formed partnership owned by Cintas           42.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_ShreditPartnershipMember
Equity method investment, recorded at fair value           339,400,000us-gaap_EquityMethodInvestmentsFairValueDisclosure
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_ShreditPartnershipMember
Other Equity Method Investments            
Schedule of Equity Method Investments [Line Items]            
Equity method investments 16,400,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_OtherEquityMethodInvestmentsMember
  16,400,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_OtherEquityMethodInvestmentsMember
  29,700,000us-gaap_EquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_OtherEquityMethodInvestmentsMember
 
Dividends received on equity method investment     5,247,000us-gaap_ProceedsFromEquityMethodInvestmentDividendsOrDistributionsReturnOfCapital
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_OtherEquityMethodInvestmentsMember
     
Cash received from sale of equity method investment     35,200,000ctas_EquityMethodInvestmentGrossTransactionProceeds
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_OtherEquityMethodInvestmentsMember
     
Gain on sale of stock of an equity method investment, net of tax     $ 13,600,000ctas_EquityMethodInvestmentRealizedGainLossonDisposalNetofTax
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= ctas_OtherEquityMethodInvestmentsMember