Refers to the amount of valuation allowance associated with junior subordinated debenture.

No definition available.

Name:	amnb_ValuationAllowanceAssociatedWithJuniorSubordinatedDebenture
Namespace Prefix:	amnb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Percentage points added to the reference rate to compute the variable rate on the debt instrument.

+ References

No definition available.

+ Details

Name:	us-gaap_DebtInstrumentBasisSpreadOnVariableRate1
Namespace Prefix:	us-gaap_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Date the debt instrument was issued, in CCYY-MM-DD format.

+ 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 210.5-02.22(a))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentIssuanceDate1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

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

- Definition

Date when the debt instrument is scheduled to be fully repaid, in CCYY-MM-DD format.

+ 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 210.5-02.22(a)(2))
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_DebtInstrumentMaturityDate
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Including the current and noncurrent portions, the carrying value of junior subordinated long-term debt owed to unconsolidated subsidiary trusts as of the balance sheet date. Such debt places a lender in a lien position behind debt having a higher priority of repayment in liquidation of the entity's assets and was initially scheduled to be repaid after one year or beyond the normal operating cycle if longer.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 944
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.7-03.16)
-URI http://asc.fasb.org/extlink&oid=6879938&loc=d3e572229-122910

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.16)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

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

- Details

Name:	dei_LegalEntityAxis=amnb_MidCarolinaOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=amnb_MidCarolinaTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_LongtermDebtTypeAxis=us-gaap_JuniorSubordinatedDebtMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	dei_LegalEntityAxis=amnb_AMNBTrustOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_VariableRateAxis=us-gaap_LondonInterbankOfferedRateLIBORMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Junior Subordinated Debt - Outstanding Payables (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Sep. 30, 2011	Jul. 01, 2011
Debt Instrument [Line Items]
Principal Amount			$ 20,619
MidCarolina Trust I
Debt Instrument [Line Items]
Fair value marks associated with junior subordinated debenture	$ 833			$ 1,197
MidCarolina Trust II
Debt Instrument [Line Items]
Fair value marks associated with junior subordinated debenture	724			$ 1,021
Junior Subordinated Debt Securities
Debt Instrument [Line Items]
Principal Amount	$ 27,826	$ 27,724
Junior Subordinated Debt Securities \| AMNB Trust I
Debt Instrument [Line Items]
Date Issued	Apr. 07, 2006
Maturity Date	Jun. 30, 2036
Principal Amount	$ 20,619	$ 20,619
Junior Subordinated Debt Securities \| AMNB Trust I \| LIBOR
Debt Instrument [Line Items]
Basis Spread on Variable Rate	1.35%	1.35%
Junior Subordinated Debt Securities \| MidCarolina Trust I
Debt Instrument [Line Items]
Date Issued	Oct. 29, 2002
Maturity Date	Nov. 07, 2032
Principal Amount	$ 4,322	$ 4,265
Junior Subordinated Debt Securities \| MidCarolina Trust I \| LIBOR
Debt Instrument [Line Items]
Basis Spread on Variable Rate	3.45%	3.45%
Junior Subordinated Debt Securities \| MidCarolina Trust II
Debt Instrument [Line Items]
Date Issued	Dec. 03, 2003
Maturity Date	Oct. 07, 2033
Principal Amount	$ 2,885	$ 2,840
Junior Subordinated Debt Securities \| MidCarolina Trust II \| LIBOR
Debt Instrument [Line Items]
Basis Spread on Variable Rate	2.95%	2.95%