Note 2 - Acquisitions - Changes in Fair Value of Net Assets Acquired and Liabilities (Details) $ in Thousands	6 Months Ended
Jun. 30, 2020 USD ($)
Goodwill at December 31, 2019	$ 84,002
Goodwill at June 30, 2020	85,048
HomeTown Bankshares Corporation [Member]
Goodwill at December 31, 2019	40,130
Premises and equipment	520
Other real estate owned	254
Other liabilities	272
Goodwill at June 30, 2020	$ 41,176

This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to other liabilities not specifically disclosed acquired in connection with a business combination for which the initial accounting was incomplete.

No definition available.

+ Details

Name:	amnb_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherLiabilities
Namespace Prefix:	amnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to other real estate owned acquired in connection with a business combination for which the initial accounting was incomplete.

+ References

No definition available.

+ Details

Name:	amnb_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherRealEstateOwned
Namespace Prefix:	amnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

This element represents the amount of any measurement period adjustment (as defined) realized during the reporting period to property, plant, and equipment acquired in connection with a business combination for which the initial accounting was incomplete.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 4A
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=121647850&loc=SL65897772-128472

+ Details

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

- Definition

Amount after accumulated impairment loss of an asset representing future economic benefits arising from other assets acquired in a business combination that are not individually identified and separately recognized.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(10)(1))
-URI http://asc.fasb.org/extlink&oid=120398452&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=120320667&loc=SL49117168-202975

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 20
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=99380562&loc=d3e13770-109266

+ Details

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

- Details

Name:	us-gaap_BusinessAcquisitionAxis=amnb_HometownBanksharesCorporationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: