Loans (Allowance for Credit Losses) (Details) - USD ($) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Allowance for Credit Losses [Roll Forward]
Beginning balance	$ 45,099	$ 36,641	$ 46,859	$ 16,278
Provision for credit loss expense	(5,000)	5,900	(6,500)	11,050
Initial allowance on loans purchased with credit deterioration				0
Loans charged-off	(241)	(200)	(613)	(768)
Recoveries collected	137	90	249	218
Total ending allowance balance	39,995	42,431	39,995	42,431
Impact of ASC 326 Adoption \| Non-PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				8,767
Impact of ASC 326 Adoption \| PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				6,886
Commercial and Industrial Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance	6,248	8,814	6,445	4,799
Provision for credit loss expense	(196)	(31)	(218)	(166)
Initial allowance on loans purchased with credit deterioration				0
Loans charged-off	0	0	(190)	(296)
Recoveries collected	28	4	43	14
Total ending allowance balance	6,080	8,787	6,080	8,787
Commercial and Industrial Loans \| Impact of ASC 326 Adoption \| Non-PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				2,245
Commercial and Industrial Loans \| Impact of ASC 326 Adoption \| PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				2,191
Commercial Real Estate Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance	28,540	17,310	29,878	4,692
Provision for credit loss expense	(4,335)	5,049	(5,669)	10,215
Initial allowance on loans purchased with credit deterioration				0
Loans charged-off	(1)	0	(10)	0
Recoveries collected	16	10	21	14
Total ending allowance balance	24,220	22,369	24,220	22,369
Commercial Real Estate Loans \| Impact of ASC 326 Adoption \| Non-PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				3,063
Commercial Real Estate Loans \| Impact of ASC 326 Adoption \| PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				4,385
Agricultural Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance	6,462	6,485	6,756	5,315
Provision for credit loss expense	(615)	545	(909)	149
Initial allowance on loans purchased with credit deterioration				0
Loans charged-off	0	0	0	0
Recoveries collected	0	0	0	0
Total ending allowance balance	5,847	7,030	5,847	7,030
Agricultural Loans \| Impact of ASC 326 Adoption \| Non-PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				1,438
Agricultural Loans \| Impact of ASC 326 Adoption \| PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				128
Leases
Allowance for Credit Losses [Roll Forward]
Beginning balance	201	172	200	0
Provision for credit loss expense	3	30	4	97
Initial allowance on loans purchased with credit deterioration				0
Loans charged-off	0	0	0	0
Recoveries collected	0	0	0	0
Total ending allowance balance	204	202	204	202
Leases \| Impact of ASC 326 Adoption \| Non-PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				105
Leases \| Impact of ASC 326 Adoption \| PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				0
Consumer Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance	439	455	490	434
Provision for credit loss expense	85	109	67	314
Initial allowance on loans purchased with credit deterioration				0
Loans charged-off	(144)	(144)	(269)	(381)
Recoveries collected	80	76	172	188
Total ending allowance balance	460	496	460	496
Consumer Loans \| Impact of ASC 326 Adoption \| Non-PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				(59)
Consumer Loans \| Impact of ASC 326 Adoption \| PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				0
Home Equity Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance	965	977	996	200
Provision for credit loss expense	(63)	85	(94)	65
Initial allowance on loans purchased with credit deterioration				0
Loans charged-off	0	0	0	0
Recoveries collected	6	0	6	0
Total ending allowance balance	908	1,062	908	1,062
Home Equity Loans \| Impact of ASC 326 Adoption \| Non-PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				762
Home Equity Loans \| Impact of ASC 326 Adoption \| PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				35
Credit Cards
Allowance for Credit Losses [Roll Forward]
Beginning balance	162	124	150	0
Provision for credit loss expense	108	26	167	60
Initial allowance on loans purchased with credit deterioration				0
Loans charged-off	(95)	(25)	(142)	(60)
Recoveries collected	6	0	6	1
Total ending allowance balance	181	125	181	125
Credit Cards \| Impact of ASC 326 Adoption \| Non-PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				124
Credit Cards \| Impact of ASC 326 Adoption \| PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				0
Residential Mortgage Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance	2,082	2,304	1,944	333
Provision for credit loss expense	13	87	152	316
Initial allowance on loans purchased with credit deterioration				0
Loans charged-off	(1)	(31)	(2)	(31)
Recoveries collected	1	0	1	1
Total ending allowance balance	2,095	2,360	2,095	2,360
Residential Mortgage Loans \| Impact of ASC 326 Adoption \| Non-PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				1,594
Residential Mortgage Loans \| Impact of ASC 326 Adoption \| PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				147
Unallocated
Allowance for Credit Losses [Roll Forward]
Beginning balance	0	0	0	505
Provision for credit loss expense	0	0	0	0
Initial allowance on loans purchased with credit deterioration				0
Loans charged-off	0	0	0	0
Recoveries collected	0	0	0	0
Total ending allowance balance	$ 0	$ 0	$ 0	0
Unallocated \| Impact of ASC 326 Adoption \| Non-PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				(505)
Unallocated \| Impact of ASC 326 Adoption \| PCD Loans
Allowance for Credit Losses [Roll Forward]
Beginning balance				$ 0

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

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

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=124255206&loc=SL82895884-210446

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossExcludingAccruedInterest
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of increase in allowance for credit loss on financing receivable purchased with credit deterioration.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossPurchasedWithCreditDeteriorationIncrease
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount of increase in allowance for credit loss on financing receivable from recovery.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=124267575&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953423-111524

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesRecovery
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of credit loss expense (reversal of expense) for financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=124255953&loc=SL82919249-210447

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04(11))
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

+ Details

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

- Details

Name:	srt_CumulativeEffectPeriodOfAdoptionAxis=srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetOtherThanFinancialAssetAcquiredWithCreditDeteriorationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_CreditLossStatusAxis=us-gaap_FinancialAssetAcquiredWithCreditDeteriorationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=gabc_CommercialAndIndustrialMember
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:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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