Commitments and Off-balance Sheet Items - Additional Information (Detail)	12 Months Ended
Dec. 31, 2012
Minimum [Member]
Commitments and Contingent Liabilities [Line Items]
Loans and Leases Receivable Fixed Commitment Interest Rates	2.25%
Maximum [Member]
Commitments and Contingent Liabilities [Line Items]
Loans and Leases Receivable Fixed Commitment Interest Rates	18.00%

Percentage of loans and leases receivable fixed commitment interest rates during the period.

No definition available.

Name:	gabc_LoansAndLeasesReceivableFixedCommitmentInterestRates
Namespace Prefix:	gabc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration