Accumulated Other Comprehensive Income (Loss) - Reclassifications Out of Accumulated Other Comprehensive Income (Loss) (Details) - USD ($) $ in Thousands	12 Months Ended
May. 31, 2015	May. 31, 2014
Equity [Abstract]
Amortization of interest rate locks	$ (3,130)	$ (3,130)
Tax benefit	1,178	1,178
Amortization of interest rate locks, net of tax	$ (1,952)	$ (1,952)

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