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Loans (Carrying Amount of Loans with Deterioration of Credit Quality) (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Financing Receivable, Recorded Investment [Line Items]    
Loans Acquired with Deteriorated Credit Quality $ 7,966us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality $ 8,361us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
Carrying Amount, Net of Allowance 7,912gabc_FinancingReceivableAcquiredWithDeterioratedCreditQualityNet 8,307gabc_FinancingReceivableAcquiredWithDeterioratedCreditQualityNet
Commercial and Industrial Loans and Leases    
Financing Receivable, Recorded Investment [Line Items]    
Loans Acquired with Deteriorated Credit Quality 396us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gabc_CommercialAndIndustrialLoansandLeasesMember		 354us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= gabc_CommercialAndIndustrialLoansandLeasesMember
Commercial Real Estate Loans    
Financing Receivable, Recorded Investment [Line Items]    
Loans Acquired with Deteriorated Credit Quality 6,070us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember		 6,385us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_CommercialRealEstatePortfolioSegmentMember
Home Equity Loans    
Financing Receivable, Recorded Investment [Line Items]    
Loans Acquired with Deteriorated Credit Quality 0us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember		 0us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_HomeEquityMember
Consumer Loans    
Financing Receivable, Recorded Investment [Line Items]    
Loans Acquired with Deteriorated Credit Quality 0us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember		 118us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ConsumerLoanMember
Residential Mortgage Loans    
Financing Receivable, Recorded Investment [Line Items]    
Loans Acquired with Deteriorated Credit Quality $ 1,500us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember		 $ 1,504us-gaap_FinancingReceivableAcquiredWithDeterioratedCreditQuality
/ us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis
= us-gaap_ResidentialMortgageMember