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
	Nov. 30, 2021	Nov. 30, 2020
Earnings Per Share [Abstract]
Net earnings (loss) attributable to Greenbrier	$ 10.8	$ (10.0)
Weighted average diluted common shares outstanding	33,570	32,723
Diluted earnings (loss) per share	$ 0.32	$ (0.30)