Stock-Based Compensation Assumptions used to determine fair value of options (Details)	12 Months Ended
May 31, 2012 Years	May 31, 2011 Years	May 31, 2010 Years
Share-based Compensation [Abstract]
Risk-fee interest rate	2.40%	2.50%	3.90%
Dividend yield	1.70%	1.50%	1.30%
Expected volatility of Cintas' common stock	37.00%	30.00%	30.00%
Expeced life of the option in years	7.5	7.5	7.5

The period of time an equity-based award is expected to be outstanding. An equity-based award's expected term is generally determined based on, among other factors, the instrument's contractual term and the effects of employees' expected exercise and post-vesting employment termination behavior.

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The estimated measure of the percentage by which a share price is expected to fluctuate during a period. Volatility also may be defined as a probability-weighted measure of the dispersion of returns about the mean. The volatility of a share price is the standard deviation of the continuously compounded rates of return on the share over a specified period. That is the same as the standard deviation of the differences in the natural logarithms of the stock prices plus dividends, if any, over the period.

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