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, 2024	May 31, 2023	May 31, 2024	May 31, 2023
Earnings Per Share Disclosure [Line Items]
Net earnings attributable to Greenbrier		$ 33.9	$ 21.3	$ 98.5	$ 37.7
Weighted average basic common shares outstanding		31,131	31,757	31,091	32,346
Basic earnings per share		$ 1.09	$ 0.67	$ 3.17	$ 1.17
Net Income (Loss)		$ 33.9	$ 21.3	$ 98.5	$ 37.7
Interest and debt issuance costs on the 2.875% convertible notes due 2024, net of tax			0.3	0.5
Earnings before interest and debt issuance costs on the 2.875% convertible notes due 2024			$ 21.6	$ 99.0
Weighted average diluted common shares outstanding		32,021	33,571	32,456	33,344
Diluted earnings per common share	[1]	$ 1.06	$ 0.64	$ 3.05	$ 1.13

[1] 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