Earnings Per Share - Approach to Calculate Diluted Earnings Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions		3 Months Ended		6 Months Ended
		Feb. 29, 2024	Feb. 28, 2023	Feb. 29, 2024	Feb. 28, 2023
Earnings Per Share Disclosure [Line Items]
Net earnings attributable to Greenbrier		$ 33.4	$ 33.1	$ 64.6	$ 16.4
Weighted average basic common shares outstanding		31,117	32,588	31,071	32,654
Basic earnings per share		$ 1.08	$ 1.01	$ 2.08	$ 0.5
Net Income (Loss)		$ 33.4	$ 33.1	$ 64.6	$ 16.4
Interest and debt issuance costs on the 2.875% convertible notes due 2024, net of tax		0.2	0.3	0.5
Earnings before interest and debt issuance costs on the 2.875% convertible notes due 2024		$ 33.6	$ 33.4	$ 65.1
Weighted average diluted common shares outstanding		32,570	34,400	32,676	33,654
Diluted earnings per common share	[1]	$ 1.03	$ 0.97	$ 1.99	$ 0.49

[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