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Stock Repurchase Plan (Additional Information) (Details)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Apr. 26, 2001
Stockholders' Equity Note [Abstract]      
Common stock, authorized shares repurchase     607,754us-gaap_StockRepurchaseProgramNumberOfSharesAuthorizedToBeRepurchased
Common stock, total shares repurchased 334,965gabc_StockRepurchasedDuringPeriodAggregateTotal    
Common stock, shares purchased 0us-gaap_StockRepurchasedDuringPeriodShares 0us-gaap_StockRepurchasedDuringPeriodShares