Long-term Borrowings (Details) $ in Thousands	3 Months Ended
Mar. 31, 2018 USD ($)
Long term advances with federal home loan bank [Abstract]
Percentage of entity's assets equal to line of credit facility	30.00%
Collateral pledged under blanket floating lien agreement	$ 561,422
Public deposit accounts	252,535
Federal Home Loan Bank Advances
Long term advances with federal home loan bank [Abstract]
Outstanding letters of credit	190,000
US government and agency securities
Long term advances with federal home loan bank [Abstract]
Outstanding letters of credit	$ 103,992

Refers to the percentage of company's assets equal to line of credit facility with federal home loan bank.

No definition available.

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Namespace Prefix:	amnb_
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- Definition

Public deposit means to accept fund from public in the form of deposit.

+ References

No definition available.

+ Details

Name:	amnb_PublicDepositAccounts
Namespace Prefix:	amnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- References

No definition available.

+ Details

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- Definition

Amount of securities, mortgage loans, or other financial instruments pledged as collateral for deposits.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 405
-Section 50
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=50488274&loc=d3e64164-112818

+ Details

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

- Definition

Amount of the collateral pledged in support of federal home loan bank advances outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=6479118&loc=d3e64650-112822

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 470
-Section 50
-Paragraph 3
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=75038535&loc=d3e64711-112823

+ Details

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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