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Derivative Instruments - Additional Information (Detail) (USD $)
In Millions, unless otherwise specified
 3 Months Ended
Nov. 30, 2014
Derivative [Line Items]  
Unrealized Pre-tax gain (loss) that would be reclassified to interest expense in the next 12 months $ 1.8us-gaap_DerivativeInstrumentsGainLossReclassificationFromAccumulatedOCIToIncomeEstimatedNetAmountToBeTransferred
Foreign Currency Forward Exchange Contracts  
Derivative [Line Items]  
Aggregate derivative notional amount 100.4invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= gbx_ForeignCurrencyForwardExchangeContractsMember
Interest rate swap contracts  
Derivative [Line Items]  
Aggregate derivative notional amount $ 98.3invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= us-gaap_InterestRateSwapMember
Maturity date 2020-03