Earnings Per Share - Approach to Calculate Diluted Earning Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		12 Months Ended
		Aug. 31, 2023		Aug. 31, 2022	Aug. 31, 2021
Earnings Per Share Disclosure [Line Items]
Net earnings attributable to Greenbrier		$ 62.5		$ 46.9	$ 32.4
Weighted average basic common shares outstanding	[1]	31,983		32,569	32,648
Basic earnings per share		$ 1.95		$ 1.44	$ 0.99
Net earnings attributable to Greenbrier		$ 62.5		$ 46.9	$ 32.4
Weighted average diluted common shares outstanding		33,799		33,631	33,665
Diluted earnings per share		$ 1.89	[2]	$ 1.40	$ 0.96
2.875% Convertible senior notes, due 2024
Earnings Per Share Disclosure [Line Items]
Interest and debt issuance costs on convertible notes, net of tax		$ 1.2

[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. No participating securities are included in basic common shares outstanding for the year ended August 31, 2023.

[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