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Related Party Transactions - Additional Information (Detail) - USD ($)	3 Months Ended		6 Months Ended
	Feb. 28, 2021	Feb. 29, 2020	Feb. 28, 2021	Feb. 29, 2020
Related Party Transaction [Line Items]
Carrying amount of investment in unconsolidated affiliates	$ 3,600,000		$ 3,600,000
Axis LLC
Related Party Transaction [Line Items]
Percentage of ownership in entity	41.90%		41.90%
Axis LLC \| Railcar Components
Related Party Transaction [Line Items]
Purchases of goods from related party	$ 2,200,000	$ 3,200,000	$ 6,000,000.0	$ 7,200,000
Greenbrier
Related Party Transaction [Line Items]
Percentage of ownership in entity	40.00%		40.00%
Greenbrier \| Leasing Warehouse
Related Party Transaction [Line Items]
Revenue recognize from railcars sold			$ 0	$ 0