Additional Collateral Based On The Value Of The Underlying Securities (Detail) (USD $) In Thousands, unless otherwise specified	12 Months Ended
Dec. 31, 2012	Dec. 31, 2011
Federal Home Loan Bank, Advances, Branch of FHLB Bank [Line Items]
Average Daily Balance During the Year	$ 19,813	$ 34,243
Average Interest Rate During the Year	0.23%	0.31%
Maximum Month-end Balance During the Year	$ 24,220	$ 43,514
Weighted Average Interest Rate at Year-end	2.07%

The calculated average interest rate for Federal Home Loan Bank advances for the period ended as of the Balance Sheet date.

No definition available.

+ Details

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

- Details

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

- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, average borrowings during the period.

+ References

No definition available.

+ Details

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

- Definition

For the form of debt having an initial term of less than one year or less than the normal operating cycle, if longer, maximum month-end borrowings during the period.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Industry Guide

-Number 3

-Section VII

-Subsection 2

+ Details

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

- Definition

Reflects the calculation as of the balance sheet date of the average interest rate weighted by the amount of short-term debt outstanding by type or by instrument at that time.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19

-Subparagraph b

-Article 5

+ Details

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Balance Type:	na
Period Type:	instant