Cash credit entitlements as a percent of grant.

No definition available.

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

- Definition

Restricted stock as a percent of grant.

+ References

No definition available.

+ Details

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

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Number of Installments

+ References

No definition available.

+ Details

Name:	gabc_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofInstallments
Namespace Prefix:	gabc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Number of Plans

+ References

No definition available.

+ Details

Name:	gabc_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofPlans
Namespace Prefix:	gabc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Offering Period

+ References

No definition available.

+ Details

Name:	gabc_SharebasedCompensationArrangementbySharebasedPaymentAwardOfferingPeriod
Namespace Prefix:	gabc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Share-based Compensation Arrangement by Share-based Payment Award, Purchase Price Percentage, Offering Date

+ References

No definition available.

+ Details

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

- Definition

The Unrecognized expense associated with the restricted stock grants and cash entitlements.

+ References

No definition available.

+ Details

Name:	gabc_UnrecognizedStockExpensesInRestrictedStock
Namespace Prefix:	gabc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of expense for award under share-based payment arrangement. Excludes amount capitalized.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5047-113901

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SAB Topic 14.F)
-URI http://asc.fasb.org/extlink&oid=115993241&loc=d3e301413-122809

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (h)(1)(i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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

- Definition

Amount of cost not yet recognized for nonvested award under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (i)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

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

- Definition

Estimated period over which an employee is required to provide service in exchange for the equity-based payment award, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of non-vested equity-based payment instruments, excluding stock (or unit) options, that validly exist and are outstanding as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(2)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
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

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

- Definition

Number of shares authorized for issuance under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The difference between the maximum number of shares (or other type of equity) authorized for issuance under the plan (including the effects of amendments and adjustments), and the sum of: 1) the number of shares (or other type of equity) already issued upon exercise of options or other equity-based awards under the plan; and 2) shares (or other type of equity) reserved for issuance on granting of outstanding awards, net of cancellations and forfeitures, if applicable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

Gross number of share options (or share units) granted during the period.

+ References

No definition available.

+ Details

Name:	us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Definition

Percentage of vesting of award under share-based payment arrangement.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=116856206&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Total number of shares issued during the period, including shares forfeited, as a result of Restricted Stock Awards.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section 50
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=109259400&loc=d3e21463-112644

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 505
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.3-04)
-URI http://asc.fasb.org/extlink&oid=27012166&loc=d3e187085-122770

+ Details

Name:	us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardGross
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockOptionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gabc_LongTermEquityIncentivePlan2009Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_RestrictedStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_TitleOfIndividualAxis=srt_DirectorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VestingAxis=us-gaap_ShareBasedCompensationAwardTrancheThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gabc_LongTermEquityIncentivePlan2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gabc_EmployeeStockPurchasePlan2009Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_AwardTypeAxis=us-gaap_EmployeeStockMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PlanNameAxis=gabc_EmployeeStockPurchasePlan2019Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_SubsequentEventTypeAxis=us-gaap_SubsequentEventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Equity Plans and Equity Based Compensation (Additional Information) (Details)			3 Months Ended		9 Months Ended
	Oct. 01, 2019 shares	Aug. 16, 2019	Sep. 30, 2019 USD ($) Plan shares	Sep. 30, 2018 USD ($) shares	Sep. 30, 2019 USD ($) Plan installment shares	Sep. 30, 2018 USD ($) shares	Dec. 31, 2018 shares
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Share-based compensation arrangement by share-based payment award, number of plans \| Plan			2		2
Number of options granted (in shares)			0	0	0	0
Stock compensation expense \| $			$ 167,000	$ 155,000	$ 509,000	$ 495,000
Unrecognized stock expenses in restricted stock \| $					2,356,000	2,277,000
Stock Options
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Stock compensation expense \| $			0	0	0	0
Unrecognized compensation expense \| $			$ 0	$ 0	$ 0	$ 0
2009 LTIP Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for grant (in shares)			0		0
Stock granted during period percentage					60.00%
Stock cash credit entitlement percentage					40.00%
Restricted stock granted during period (in shares)			5,466	180	31,329	35,490
2009 LTIP Plan \| Restricted Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of annual installments \| installment					3
Unvested restricted stock awards (in shares)			76,011		76,011		44,682
2009 LTIP Plan \| Restricted Stock \| Director
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested in period percentage					100.00%
Required service period for directors after grant					1 year
2009 LTIP Plan \| Restricted Stock \| Installment 1
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested in period percentage					33.30%
2009 LTIP Plan \| Restricted Stock \| Installment 2
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested in period percentage					33.30%
2009 LTIP Plan \| Restricted Stock \| Installment 3
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Vested in period percentage					33.30%
2019 LTIP Plan
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for grant (in shares)			994,534		994,534
Number of shares authorized for issuance (in shares)			1,000,000		1,000,000
2009 ESPP \| Employee Stock
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares authorized for issuance (in shares)			750,000		750,000
Stock compensation expense \| $			$ 30,000	$ 30,000	$ 39,000	$ 39,000
Unrecognized compensation expense \| $			$ 0	$ 0	$ 0	$ 0
Purchase price percentage		95.00%
2019 ESPP \| Subsequent Event
Share-based Compensation Arrangement by Share-based Payment Award [Line Items]
Number of shares reserved for grant (in shares)	750,000
Purchase price percentage	95.00%
Offering periods	3 months