Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of Accumulated Other Comprehensive (Loss) Income (Details) - USD ($) $ in Thousands	3 Months Ended
Aug. 31, 2015	Aug. 31, 2014
Equity [Abstract]
Amortization of interest rate locks	$ (783)	$ (783)
Tax benefit	295	295
Amortization of interest rate locks, net of tax	$ (488)	$ (488)

Other Comprehensive Income (Loss), Amortization of Interest Rate Lock Agreements, before Tax

No definition available.

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