Interim Financial Statements - Additional Information (Detail) - USD ($) $ in Millions	Feb. 28, 2022	Sep. 01, 2021	Aug. 31, 2021	Jun. 01, 2017
Equity Class Of Treasury Stock [Line Items]
Increase in retained earnings	$ 892.5		$ 881.7
Reduction to additional paid in capital	$ (413.4)		$ (469.7)
ASU 2020-06
Equity Class Of Treasury Stock [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Sep. 01, 2021
ASU 2020-06 | Cumulative Effect Adjustment Due to Adoption
Equity Class Of Treasury Stock [Line Items]
Increase in retained earnings		$ 5.0
Reduction to additional paid in capital		(59.0)
Derecognition of deferred tax liabilities		17.0
Increase in convertible note due to derecognition of debt discount		$ 71.0
ASU 2019-12
Equity Class Of Treasury Stock [Line Items]
Change in Accounting Principle, Accounting Standards Update, Adopted [true false]	true
Change in Accounting Principle, Accounting Standards Update, Immaterial Effect [true false]	true
Change in Accounting Principle, Accounting Standards Update, Adoption Date	Sep. 01, 2021
Greenbrier-Astra Rail
Equity Class Of Treasury Stock [Line Items]
Ownership percentage by parent				75.00%