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Revenue Recognition - Additional information (Detail) - USD ($) $ in Millions	12 Months Ended
	Aug. 31, 2025	Aug. 31, 2024
Revenue From Contract With Customers [Line Items]
Revenue recognized from contract with customers liability	$ 39.9	$ 22.9

X

- Definition

Revenue From Contract With Customers [Line Items]

+ References

No definition available.

+ Details

Name:	gbx_RevenueFromContractWithCustomersLineItems
Namespace Prefix:	gbx_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Definition

Amount of revenue recognized that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-8

+ Details

Name:	us-gaap_ContractWithCustomerLiabilityRevenueRecognized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration