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Derivative Instruments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 6 Months Ended
Feb. 28, 2015
Derivative [Line Items]  
Unrealized Pre-tax gain (loss) that would be reclassified to interest expense in the next 12 months $ 1.7us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred
Foreign Currency Forward Exchange Contracts  
Derivative [Line Items]  
Aggregate derivative notional amount 86.8invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= gbx_ForeignCurrencyForwardExchangeContractsMember
Interest rate swap contracts  
Derivative [Line Items]  
Aggregate derivative notional amount $ 97.4invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
Maturity date 2020-03