Lessor and lessee lease, description.

No definition available.

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

- Definition

Operating Lease Remaining Lease Term.

+ References

No definition available.

+ Details

Name:	gbx_OperatingLeaseRemainingLeaseTerm
Namespace Prefix:	gbx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Sublease period.

+ References

No definition available.

+ Details

Name:	gbx_SubleasePeriod
Namespace Prefix:	gbx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of accumulated depreciation, depletion and amortization for physical assets used in the normal conduct of business to produce goods and services.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.14)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

The amount of expense recognized in the current period that reflects the allocation of the cost of tangible assets over the assets' useful lives. Includes production and non-production related depreciation.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 1
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-URI http://asc.fasb.org/extlink&oid=6391035&loc=d3e2868-110229

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Description of terms and conditions of option to extend lessee's operating lease. Includes, but is not limited to, information about option recognized as part of right-of-use asset and lease liability.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 20
-Section 50
-Paragraph 3
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=77888426&loc=SL77918673-209980

+ Details

Name:	us-gaap_LesseeOperatingLeaseOptionToExtend
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of operating lease income from lease payments and variable lease payments paid and payable to lessor. Includes, but is not limited to, variable lease payments not included in measurement of lease receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 842
-SubTopic 30
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=119202524&loc=SL77919372-209981

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-Publisher FASB
-Name Accounting Standards Codification
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-URI http://asc.fasb.org/extlink&oid=77888421&loc=SL77919311-209978

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 270
-SubTopic 10
-Section 50
-Paragraph 6A
-URI http://asc.fasb.org/extlink&oid=118952077&loc=SL77927221-108306

+ Details

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

- Details

Name:	srt_ProductOrServiceAxis=gbx_CarHireUtilizationArrangementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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Namespace Prefix:
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Balance Type:
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- Details

Name:	us-gaap_PropertySubjectToOrAvailableForOperatingLeaseAxis=us-gaap_PropertySubjectToOperatingLeaseMember
Namespace Prefix:
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Leases - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
	Feb. 29, 2020	Feb. 29, 2020	Aug. 31, 2019
Lessor and Lessee, Lease, Description [Line Items]
Operating lease rental revenues	$ 9.8	$ 21.2
Finance lease option to extend		15
Car Hire Utilization Arrangements
Lessor and Lessee, Lease, Description [Line Items]
Operating lease rental revenues	2.8	$ 6.5
Minimum
Lessor and Lessee, Lease, Description [Line Items]
Sublease period		1 year
Financing lease remaining lease term		1 year
Maximum
Lessor and Lessee, Lease, Description [Line Items]
Sublease period		5 years
Financing lease remaining lease term		79 years
Property Subject to Operating Lease
Lessor and Lessee, Lease, Description [Line Items]
Accumulated depreciation	32.9	$ 32.9	$ 44.2
Depreciation expense	$ 3.0	$ 6.6