Earnings Per Share - Approach to Calculate Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended		9 Months Ended
		May 31, 2022	May 31, 2021	May 31, 2022	May 31, 2021
Earnings Per Share Disclosure [Line Items]
Net earnings attributable to Greenbrier		$ 3.1	$ 19.7	$ 26.7	$ 0.7
Weighted average basic common shares outstanding	[1]	32,588	32,573	32,560	32,726
Basic earnings per share		$ 0.10	$ 0.61	$ 0.82	$ 0.02
Net earnings attributable to Greenbrier		$ 3.1	$ 19.7	$ 26.7	$ 0.7
Weighted average diluted common shares outstanding		33,661	33,605	33,626	33,747
Diluted earnings per share		$ 0.09	$ 0.59	$ 0.79	$ 0.02

[1] Restricted stock grants and restricted stock units that are considered participating securities, including some grants subject to certain performance criteria, are included in weighted average basic common shares outstanding when the Company is in a net earnings position.