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Equity Plans and Equity Based Compensation (Additional Information) (Details) (USD $)
3 Months Ended
Mar. 31, 2015
installment
Plan
Mar. 31, 2014
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Share-based compensation arrangement by share-based payment award, number of plans 3gabc_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofPlans  
Number of shares available for grant 390,033us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant  
Stock compensation expense $ 162,000us-gaap_AllocatedShareBasedCompensationExpense $ 99,000us-gaap_AllocatedShareBasedCompensationExpense
Stock granted during period percentage 60.00%gabc_ScheduleOfShareBasedCompensationRestrictedStockAndRestrictedStockGrantDuringPeriodPercentage  
Stock cash credit entitlement percentage 40.00%gabc_ScheduleOfShareBasedCompensationRestrictedStockAndRestrictedStockCashCreditEntitlementPercentage  
Number of equal installments 3gabc_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofInstallments  
Vested in period percentage 33.33%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage  
Stock issued during period 32,610us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures 31,080us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures
Unrecognized stock expenses in restricted stock 2,615,000gabc_UnrecognizedStockExpensesInRestrictedStock 2,127,000gabc_UnrecognizedStockExpensesInRestrictedStock
Discount from market price, offering date 95.00%us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardDiscountFromMarketPriceOfferingDate  
Director    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Vested in period percentage 100.00%us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
/ us-gaap_TitleOfIndividualAxis
= us-gaap_DirectorMember
 
Employee Stock Option    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of options granted 0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
0us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Expense of stock options granted 0us-gaap_StockOptionPlanExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
0us-gaap_StockOptionPlanExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Stock compensation expense 0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Unrecognized option expense 0us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
0us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockOptionMember
Employee Stock    
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]    
Number of shares available for grant 500,000us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
 
Stock compensation expense 0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
0us-gaap_AllocatedShareBasedCompensationExpense
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
Unrecognized option expense $ 0us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember
$ 0us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized
/ us-gaap_AwardTypeAxis
= us-gaap_EmployeeStockMember