Earnings Per Share - Approach to Calculate Diluted Earning Per Share (Detail) - USD ($) $ / shares in Units, shares in Thousands, $ in Thousands	3 Months Ended																12 Months Ended
	Aug. 31, 2018		May 31, 2018		Feb. 28, 2018		Nov. 30, 2017		Aug. 31, 2017		May 31, 2017		Feb. 28, 2017		Nov. 30, 2016		Aug. 31, 2018		Aug. 31, 2017		Aug. 31, 2016
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Net earnings attributable to Greenbrier	$ 30,935		$ 32,951		$ 61,642		$ 26,253		$ 23,746		$ 32,823		$ 34,536		$ 24,962		$ 151,781		$ 116,067		$ 183,213
Earnings before interest and debt issuance costs on convertible notes																	$ 153,812		$ 118,999		$ 185,908
Weighted average diluted common shares outstanding																	32,835		32,562		32,468
Diluted earnings per share	$ 0.94	[1]	$ 1.01	[1]	$ 1.91	[1]	$ 0.83	[1]	$ 0.75	[2]	$ 1.03	[2]	$ 1.09	[2]	$ 0.79	[2]	$ 4.68	[1],[3]	$ 3.65	[2],[3]	$ 5.73	[3]
2018 Senior Notes
Earnings Per Share, Diluted, by Common Class, Including Two Class Method [Line Items]
Interest and debt issuance costs on the 2018 Convertible notes, net of tax																	$ 2,031		$ 2,932		$ 2,695

[1] Quarterly amounts do not total to the year to date amount as each period is calculated discretely. Diluted earnings per common share includes the dilutive effect of the 2024 Convertible Notes using the treasury stock method when dilutive, restricted stock units that are not considered participating securities, restricted stock units that are subject to performance criteria for which actual levels of performance above target have been achieved and the dilutive effect of shares underlying the 2018 Convertible Notes, during the periods in which they were outstanding, using the "if converted" method in which debt issuance and interest costs, net of tax, were added back to net earnings. The 2018 Convertible notes matured on April 1, 2018.

[2] Quarterly amounts do not total to the year to date amount as each period is calculated discretely. Diluted earnings per common share includes the dilutive effect of the 2024 Convertible Notes using the treasury stock method when dilutive, restricted stock units that are subject to performance criteria for which actual levels of performance above target have been achieved and the dilutive effect of shares underlying the 2018 Convertible Notes using the "if converted" method in which debt issuance and interest costs, net of tax, were added back to net earnings.

[3] Diluted earnings per share was calculated as follows: Earnings before interest and debt issuance costs on convertible notes Weighted average diluted common shares outstanding