Loans - Narrative (Details) $ in Thousands	3 Months Ended	12 Months Ended
Mar. 31, 2024 USD ($) loan	Mar. 31, 2023 loan	Dec. 31, 2023 USD ($) loan
Accounts, Notes, Loans and Financing Receivable [Line Items]
Interest income on non-accrual loans	$ 32		$ 106
Number of loans modified as troubled debt restructurings \| loan	0	0	0
Threshold amount to individually classify loans by credit risk	$ 250
PCD Loans
Accounts, Notes, Loans and Financing Receivable [Line Items]
Purchase credit deteriorated loans	$ 12,817		$ 13,237

Financing Receivable, Individually Evaluated For Impairment, Threshold Amount

No definition available.

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Namespace Prefix:	gabc_
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Balance Type:	debit
Period Type:	instant

- Definition

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.

+ References

No definition available.

+ Details

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Namespace Prefix:	us-gaap_
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Balance Type:	na
Period Type:	duration

- Definition

Amount, after allowance for credit loss, of accrued interest on financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 5
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479344/326-20-45-5

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 3A
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-3A

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 310
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 33
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147481962/310-10-50-33

+ Details

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

- Definition

Amount of interest income on financing receivable on nonaccrual status.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 326
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 16
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org//1943274/2147479319/326-20-50-16

+ Details

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Balance Type:	credit
Period Type:	duration

- Details

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Data Type:	na
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