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FHLB Advances and Other Borrowings (Schedule of Issuance of Subordinated Debentures) (Details) (USD $)
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
May 06, 2005
Jul. 15, 2005
Debt Instrument [Line Items]        
Percentage of Variable Rate 2.875%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1 2.875%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1    
ACB Trust I        
Debt Instrument [Line Items]        
Date of Issuance May 06, 2005      
Issuance Amount     5,155,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= gabc_AcbTrustOneMember
 
Carrying Amount at December 31, 2014 3,278,000us-gaap_DebtInstrumentFairValue
/ us-gaap_DebtInstrumentAxis
= gabc_AcbTrustOneMember
     
Variable Rate 90 day LIBOR + 2.15%      
Rate as of December 31, 2013 2.41%us-gaap_DebtInstrumentInterestRateDuringPeriod
/ us-gaap_DebtInstrumentAxis
= gabc_AcbTrustOneMember
2.40%us-gaap_DebtInstrumentInterestRateDuringPeriod
/ us-gaap_DebtInstrumentAxis
= gabc_AcbTrustOneMember
   
Maturity Date May 31, 2035      
ACB Trust II        
Debt Instrument [Line Items]        
Date of Issuance Jul. 15, 2005      
Issuance Amount       3,093,000us-gaap_DebtInstrumentFaceAmount
/ us-gaap_DebtInstrumentAxis
= gabc_AcbTrustTwoMember
Carrying Amount at December 31, 2014 1,896,000us-gaap_DebtInstrumentFairValue
/ us-gaap_DebtInstrumentAxis
= gabc_AcbTrustTwoMember
     
Variable Rate 90 day LIBOR + 1.85%      
Rate as of December 31, 2013 2.08%us-gaap_DebtInstrumentInterestRateDuringPeriod
/ us-gaap_DebtInstrumentAxis
= gabc_AcbTrustTwoMember
2.09%us-gaap_DebtInstrumentInterestRateDuringPeriod
/ us-gaap_DebtInstrumentAxis
= gabc_AcbTrustTwoMember
   
Maturity Date Jul. 31, 2035      
London Interbank Offered Rate (LIBOR) [Member] | ACB Trust I        
Debt Instrument [Line Items]        
Percentage of Variable Rate     2.15%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= gabc_AcbTrustOneMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember
 
London Interbank Offered Rate (LIBOR) [Member] | ACB Trust II        
Debt Instrument [Line Items]        
Percentage of Variable Rate       1.85%us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
/ us-gaap_DebtInstrumentAxis
= gabc_AcbTrustTwoMember
/ us-gaap_VariableRateAxis
= us-gaap_LondonInterbankOfferedRateLIBORMember