Earnings (Loss) Per Share - Approach to Calculate Diluted Earning (Loss) Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended		6 Months Ended
	Feb. 28, 2021	Feb. 29, 2020	Feb. 28, 2021	Feb. 29, 2020
Earnings Per Share [Abstract]
Net earnings (loss) attributable to Greenbrier	$ (9,065)	$ 13,629	$ (19,037)	$ 21,298
Weighted average diluted common shares outstanding	32,810	33,482	32,766	33,382
Diluted earnings (loss) per common share	$ (0.28)	$ 0.41	$ (0.58)	$ 0.64