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

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

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://fasb.org/us-gaap/role/ref/legacyRef
-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://fasb.org/us-gaap/role/ref/legacyRef
-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	9 Months Ended
	Sep. 30, 2018	Dec. 31, 2017	Jul. 01, 2011	Apr. 07, 2006
Debt Instrument [Line Items]
Principal Amount				$ 20,619
MidCarolina Trust I
Debt Instrument [Line Items]
Valuation allowance associated with junior subordinated debenture	$ 1,481		$ 1,197
MidCarolina Trust II
Debt Instrument [Line Items]
Valuation allowance associated with junior subordinated debenture		$ 1,557	$ 1,021
Junior Subordinated Debt Securities
Debt Instrument [Line Items]
Principal Amount	27,902	27,826
Junior Subordinated Debt Securities \| AMNB Trust I
Debt Instrument [Line Items]
Principal Amount	20,619	20,619
Junior Subordinated Debt Securities \| MidCarolina Trust I
Debt Instrument [Line Items]
Principal Amount	4,363	4,322
Junior Subordinated Debt Securities \| MidCarolina Trust II
Debt Instrument [Line Items]
Principal Amount	$ 2,920	$ 2,885
Junior Subordinated Debt Securities \| LIBOR \| AMNB Trust I
Debt Instrument [Line Items]
Basis spread on variable rate	1.35%
Junior Subordinated Debt Securities \| LIBOR \| MidCarolina Trust I
Debt Instrument [Line Items]
Basis spread on variable rate	3.45%
Junior Subordinated Debt Securities \| LIBOR \| MidCarolina Trust II
Debt Instrument [Line Items]
Basis spread on variable rate	2.95%