XML 111 R90.htm IDEA: XBRL DOCUMENT
v3.21.2

Equity - Summary of Restricted Stock Share and Restricted Stock Unit Grant Transactions for Shares, both Vested and Unvested (Detail) - shares | 12 Months Ended
--- | ---
Aug. 31, 2021 | Aug. 31, 2020 | Aug. 31, 2019
Equity [Abstract] |  |  |  |
Beginning Balance | [1] | 4,959,364 | 4,575,478 | 4,374,325
Granted |  | 538,260 | 469,825 | 313,540
Forfeited |  | (190,854) | (85,939) | (112,387)
Ending Balance | [1] | 5,306,770 | 4,959,364 | 4,575,478
[1] | Balance represents cumulative grants net of forfeitures.

X
- Definition Share-based Compensation Arrangement by Share-based Payment Award, Equity Instruments Other than Options, Vested and Unvested, Number + References No definition available. + Details
Name: | gbx_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedAndUnvestedNumber
Namespace Prefix: | gbx_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | instant

X
- References No definition available. + Details
Name: | us-gaap_EquityAbstract
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of equity-based payment instruments, excluding stock (or unit) options, that were forfeited during the reporting period. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(1)(iv)(3) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration

X
- Definition The number of grants made during the period on other than stock (or unit) option plans (for example, phantom stock or unit plan, stock or unit appreciation rights plan, performance target plan). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 718 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c)(2)(iii)(1) -URI http://asc.fasb.org/extlink&oid=120381028&loc=d3e5070-113901 + Details
Name: | us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod
Namespace Prefix: | us-gaap_
Data Type: | xbrli:sharesItemType
Balance Type: | na
Period Type: | duration