Earnings (Loss) Per Share - Approach to Calculate Diluted Earning (Loss) Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended		6 Months Ended
		Feb. 28, 2022	Feb. 28, 2021	Feb. 28, 2022	Feb. 28, 2021
Earnings Per Share Disclosure [Line Items]
Net earnings (loss) attributable to Greenbrier		$ 12.8	$ (9.1)	$ 23.6	$ (19.1)
Weighted average basic common shares outstanding	[1]	32,582	32,810	32,546	32,766
Basic earnings (loss) per share		$ 0.39	$ (0.28)	$ 0.72	$ (0.58)
Net earnings (loss) attributable to Greenbrier		$ 12.8	$ (9.1)	$ 23.6	$ (19.1)
Weighted average diluted common shares outstanding		34,463	32,810	33,609	32,766
Diluted earnings (loss) per share		$ 0.38	$ (0.28)	$ 0.70	$ (0.58)
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		$ 13.1

[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)