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, 2023	May 31, 2022	May 31, 2023	May 31, 2022
Earnings Per Share Disclosure [Line Items]
Net earnings attributable to Greenbrier		$ 21.3	$ 3.1	$ 37.7	$ 26.7
Weighted average basic common shares outstanding	[1]	31,757	32,588	32,346	32,560
Basic earnings per share		$ 0.67	$ 0.10	$ 1.17	$ 0.82
Net earnings attributable to Greenbrier		$ 21.3	$ 3.1	$ 37.7	$ 26.7
Weighted average diluted common shares outstanding		33,571	33,661	33,344	33,626
Diluted earnings per common share	[2]	$ 0.64	$ 0.09	$ 1.13	$ 0.79
2.875% Convertible senior notes, due 2024
Earnings Per Share Disclosure [Line Items]
Interest and debt issuance costs on convertible notes, net of tax		$ 0.3
Earnings before interest and debt issuance costs on convertible notes		$ 21.6

[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.

[2] Diluted earnings per share was calculated as follows: Earnings before interest and debt issuance costs on the 2.875% convertible notes due 2024 Weighted average diluted common shares outstanding