Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableModificationsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount, after troubled debt restructuring, of modified financing receivable.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount, before troubled debt restructuring, of financing receivable to be modified.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=gabc_CommercialAndIndustrialLoansandLeasesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Loans (Schedule of Loans by Class Modified as Debt Restructuring) (Details) $ in Thousands	12 Months Ended
	Dec. 31, 2019 loan	Dec. 31, 2018 loan	Dec. 31, 2017 USD ($) loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan	0	0	3
Pre-Modification Outstanding Recorded Investment			$ 505
Post-Modification Outstanding Recorded Investment			$ 505
Commercial and Industrial Loans and Leases
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan			2
Pre-Modification Outstanding Recorded Investment			$ 477
Post-Modification Outstanding Recorded Investment			$ 477
Commercial Real Estate Loans
Financing Receivable, Troubled Debt Restructuring [Line Items]
Number of Loans \| loan			1
Pre-Modification Outstanding Recorded Investment			$ 28
Post-Modification Outstanding Recorded Investment			$ 28