Investments - Narrative (Details) - USD ($)	3 Months Ended	12 Months Ended
	Aug. 31, 2014	May. 31, 2015	May. 31, 2014	May. 31, 2013	Apr. 30, 2014
Schedule of Equity Method Investments [Line Items]
Investments		$ 329,692,000	$ 458,357,000
Cash surrender value of life insurance policies		101,800,000	86,500,000
Equity method investments		225,700,000	371,100,000
Cost method investments		2,200,000	800,000
Losses due to impairment		0	0	$ 0
Gain (loss) on investment in Shred-it, net of tax		(5,539,000)	1,234,000	0
Dividends received on equity method investment		5,247,000	0	0
Cash received from sale of equity method investment		35,200,000
Gain on sale of stock of an equity method investment, net of tax	$ 13,600,000	13,600,000
Shred-it Partnership
Schedule of Equity Method Investments [Line Items]
Equity method investments		210,100,000	341,400,000
Percentage of newly formed partnership owned by Cintas					42.00%
Equity method investment, recorded at fair value					$ 339,400,000
Difference between carrying value and underlying equity in net assets		$ 94,000,000
Amortization period of basis difference		9 years
Gain (loss) on investment in Shred-it, net of tax		$ (5,500,000)	1,200,000
Dividends received on equity method investment		113,400,000	$ 0	$ 0
Shred-it | Shred-it Partnership
Schedule of Equity Method Investments [Line Items]
Partnership, ownership percentage by third party					58.00%
(Loss) Gain on Investment in Shred-it | Shred-it Partnership
Schedule of Equity Method Investments [Line Items]
Amortization of basis difference		$ 11,000,000