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Newly Issued Accounting Pronouncements (Additional Information) (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Accrued Interest Receivable and Other Assets      
Schedule of Equity Method Investments [Line Items]      
Investments in qualified housing projects $ 5,900,000gabc_AmortizationMethodQualifiedAffordableHousingProjectInvestments
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  $ 6,100,000gabc_AmortizationMethodQualifiedAffordableHousingProjectInvestments
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Accrued Interest Payable and Other Liabilities      
Schedule of Equity Method Investments [Line Items]      
Unfunded investment in qualified affordable housing investments 4,800,000gabc_QualifiedAffordableHousingProjectInvestmentsCommitment
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  4,800,000gabc_QualifiedAffordableHousingProjectInvestmentsCommitment
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Income Tax Expense      
Schedule of Equity Method Investments [Line Items]      
Low income housing investment losses, amortization method 128,000gabc_IncomeLossfromAffordableHousingProjectsAmortizationMethodInvestments
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Low income housing investment (benefit) losses, equity method   (35,000)gabc_IncomeLossfromAffordableHousingProjectsEquityMethodInvestments
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Other Operating Expense      
Schedule of Equity Method Investments [Line Items]      
Low income housing investment (benefit) losses, equity method   20,000gabc_IncomeLossfromAffordableHousingProjectsEquityMethodInvestments
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FASB Issued Guidance For Qualified Housing Projects      
Schedule of Equity Method Investments [Line Items]      
Low income housing investment loss, due to adoption of amortization method $ 161,000gabc_AmortizationMethodQualifiedAffordableHousingProjectInvestmentsAmortization
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