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Commitments and Contingencies - Additional Information (Detail) (USD $)
6 Months Ended 12 Months Ended
Feb. 28, 2015
Aug. 31, 2014
Jul. 18, 2014
Commitments and Contingencies Disclosure [Line Items]      
Remedial investigation and feasibility study $ 110,000,000gbx_FeasibilityAndEvaluationCosts    
Remedial investigation and feasibility study, period 14 years    
Remedial investigation and feasibility study, expiration year 2017    
Bank and third party warranty and performance guarantee facilities 3,900,000gbx_WarrantLiabilitiesCurrent    
GBW Railcar Services Holdings, LLC      
Commitments and Contingencies Disclosure [Line Items]      
Equity method investment, percentage of ownership interest     50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gbx_GbwRailcarServicesHoldingsLlcMember
Payment to acquire equity method investments 3,200,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gbx_GbwRailcarServicesHoldingsLlcMember
12,500,000us-gaap_PaymentsToAcquireEquityMethodInvestments
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gbx_GbwRailcarServicesHoldingsLlcMember
 
Account receivable 21,000,000us-gaap_AccountsReceivableGross
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gbx_GbwRailcarServicesHoldingsLlcMember
   
Note receivable 2,500,000us-gaap_NotesReceivableGross
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gbx_GbwRailcarServicesHoldingsLlcMember
   
Proceeds from lease receivables 5,000,000us-gaap_ProceedsFromCollectionOfLeaseReceivables
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gbx_GbwRailcarServicesHoldingsLlcMember
   
Quarterly distributions received 700,000us-gaap_EquityMethodInvestmentDividendsOrDistributions
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gbx_GbwRailcarServicesHoldingsLlcMember
   
GBW Railcar Services Holdings, LLC | Watco Companies LLC      
Commitments and Contingencies Disclosure [Line Items]      
Equity method investment, percentage of ownership interest     50.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ dei_LegalEntityAxis
= gbx_WatcoCompaniesLlcMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gbx_GbwRailcarServicesHoldingsLlcMember
Performance Guarantee      
Commitments and Contingencies Disclosure [Line Items]      
Outstanding letters of credit 10,100,000us-gaap_LettersOfCreditOutstandingAmount
/ us-gaap_GuaranteeObligationsByNatureAxis
= us-gaap_PerformanceGuaranteeMember
   
Mexican Joint Venture Credit Facility      
Commitments and Contingencies Disclosure [Line Items]      
Debt outstanding 38,900,000gbx_DebtOutstanding
/ us-gaap_GuaranteeObligationsByNatureAxis
= gbx_MexicanJointVentureCreditFacilityMember
   
Guarantee obligations 34,400,000us-gaap_GuaranteeObligationsMaximumExposure
/ us-gaap_GuaranteeObligationsByNatureAxis
= gbx_MexicanJointVentureCreditFacilityMember
   
Minimum      
Commitments and Contingencies Disclosure [Line Items]      
Period for remedial action 2 years    
Maximum      
Commitments and Contingencies Disclosure [Line Items]      
Period for remedial action 28 years    
Maximum | GBW Railcar Services Holdings, LLC      
Commitments and Contingencies Disclosure [Line Items]      
Expected loan to unconsolidated joint venture 12,000,000gbx_ExpectedLoanOrContributionToUnconsolidatedJointVenture
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis
= gbx_GbwRailcarServicesHoldingsLlcMember
   
Portland Harbor Site | Minimum      
Commitments and Contingencies Disclosure [Line Items]      
Cleanup expenses 169,000,000gbx_CostOfEnvironmentalCleanup
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementBusinessSegmentsAxis
= gbx_LocationOneMember
   
Portland Harbor Site | Maximum      
Commitments and Contingencies Disclosure [Line Items]      
Cleanup expenses 1,800,000,000gbx_CostOfEnvironmentalCleanup
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementBusinessSegmentsAxis
= gbx_LocationOneMember
   
Area Of Willamette River Adjacent to the Company's Portland, Oregon Manufacturing Facility | Minimum      
Commitments and Contingencies Disclosure [Line Items]      
Cleanup expenses 9,000,000gbx_CostOfEnvironmentalCleanup
/ us-gaap_RangeAxis
= us-gaap_MinimumMember
/ us-gaap_StatementBusinessSegmentsAxis
= gbx_LocationTwoMember
   
Area Of Willamette River Adjacent to the Company's Portland, Oregon Manufacturing Facility | Maximum      
Commitments and Contingencies Disclosure [Line Items]      
Cleanup expenses $ 163,000,000gbx_CostOfEnvironmentalCleanup
/ us-gaap_RangeAxis
= us-gaap_MaximumMember
/ us-gaap_StatementBusinessSegmentsAxis
= gbx_LocationTwoMember