Earnings per Share (Details) (USD $) In Millions, except Share data, unless otherwise specified	3 Months Ended
	Aug. 31, 2014	Aug. 31, 2013
Earnings per Share
Effect to the participating securities as result of net loss	$ 0
Basic EPS:
Net income attributable to AAR and noncontrolling interest	14.5	18.0
Less income attributable to participating shares	(0.3)	(0.4)
Less income attributable to noncontrolling interest	(0.1)	(0.1)
Net income attributable to AAR available to common shareholders	14.1	17.5
Basic shares:
Weighted average common shares outstanding	38,800,000	38,600,000
Earnings per share - basic:
Earnings per share - basic (in dollars per share)	$ 0.36	$ 0.45
Diluted EPS:
Net income attributable to AAR and noncontrolling interest	14.5	18.0
Less income attributable to participating shares	(0.3)	(0.4)
Less income attributable to noncontrolling interest	(0.1)	(0.1)
Net income attributable to AAR available to common shareholders	$ 14.1	$ 17.5
Diluted shares:
Weighted average common shares outstanding	38,800,000	38,600,000
Additional shares from the assumed exercise of stock options	400,000	400,000
Weighted average common shares outstanding - diluted	39,200,000	39,000,000
Earnings per share - diluted:
Earnings per share - diluted (in dollars per share)	$ 0.36	$ 0.45
Antidilutive stock options excluded from the computation of diluted earnings per share (in shares)	180,163	1,211,672