No definition available.

Name:	amnb_LoanAbstract
Namespace Prefix:	amnb_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

+ References

No definition available.

+ Details

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

- Definition

Amount of loans and leases that have been written off from both loan receivables and allowance reserve for credit loss.

+ References

+ Details

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

- References

No definition available.

+ Details

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

- Definition

The valuation allowance for financing receivables that are expected to be uncollectible that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

Amount of valuation allowance for financing receivables that are expected to be uncollectible that were individually evaluated for impairment.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesIndividuallyEvaluatedForImpairment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of recovery of financing receivables doubtful of collection that were previously charged off.

+ References

+ Details

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

- Definition

The balance of financing receivables that were collectively evaluated for impairment.

+ References

+ Details

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

- Definition

The balance of financing receivables that were individually evaluated for impairment.

+ References

+ Details

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

- Definition

Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 4: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

Reference 5: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

+ Details

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

- Definition

Amount before allowance and after deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

+ Details

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

- Definition

Amount of expense related to estimated loss from loan and lease transactions.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=98514028&loc=d3e3602-108585

Reference 3: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(2)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

+ Details

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

- Details

Name:	us-gaap_ReceivablesAcquiredWithDeterioratedCreditQualityAxis=us-gaap_ReceivablesAcquiredWithDeterioratedCreditQualityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialRealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ResidentialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_UnallocatedFinancingReceivablesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Allowance for Loan Losses and Reserve for Unfunded Lending Commitments - Allowance by Portfolio Segment (Details) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2017	Dec. 31, 2016
Allowance for Loan Losses
Balance, beginning of year	$ 12,801	$ 12,601	$ 12,427
Charge-offs	(690)	(326)	(1,200)
Recoveries	476	276	424
Provision	1,016	250	950
Balance, end of year	13,603	12,801	12,601
Allowance for Loan Losses
Individually evaluated for impairment				$ 167	$ 23
Collectively evaluated for impairment				13,151	12,429
Total	12,801	12,601	12,427	13,603	12,801
Loans
Individually evaluated for impairment				3,217	4,844
Collectively evaluated for impairment				1,309,478	1,131,308
Loans, net of unearned income				1,336,125	1,164,821
Receivables Acquired with Deteriorated Credit Quality
Allowance for Loan Losses
Balance, beginning of year	349
Balance, end of year	285	349
Allowance for Loan Losses
Total	349	349		285	349
Loans
Loans, net of unearned income				23,430	28,669
Commercial
Allowance for Loan Losses
Balance, beginning of year	2,095	2,065
Charge-offs	(282)	(40)
Recoveries	223	40
Provision	377	30
Balance, end of year	2,413	2,095	2,065
Allowance for Loan Losses
Individually evaluated for impairment				154	0
Collectively evaluated for impairment				2,259	2,087
Total	2,095	2,065	2,065	2,413	2,095
Loans
Individually evaluated for impairment				206	43
Collectively evaluated for impairment				251,185	208,258
Loans, net of unearned income				251,666	208,717
Commercial \| Receivables Acquired with Deteriorated Credit Quality
Allowance for Loan Losses
Balance, beginning of year	8
Balance, end of year	0	8
Allowance for Loan Losses
Total	8	8		0	8
Loans
Loans, net of unearned income				275	416
Commercial real estate
Allowance for Loan Losses
Balance, beginning of year	7,355	6,930
Charge-offs	(93)	(10)
Recoveries	60	32
Provision	999	403
Balance, end of year	8,321	7,355	6,930
Allowance for Loan Losses
Individually evaluated for impairment				0	0
Collectively evaluated for impairment				8,203	7,248
Total	7,355	6,930	6,930	8,321	7,355
Loans
Individually evaluated for impairment				862	2,186
Collectively evaluated for impairment				747,819	610,462
Loans, net of unearned income				760,848	625,218
Commercial real estate \| Receivables Acquired with Deteriorated Credit Quality
Allowance for Loan Losses
Balance, beginning of year	107
Balance, end of year	118	107
Allowance for Loan Losses
Total	107	107		118	107
Loans
Loans, net of unearned income				12,167	12,570
Residential real estate
Allowance for Loan Losses
Balance, beginning of year	3,303	3,546
Charge-offs	(172)	(87)
Recoveries	85	68
Provision	(391)	(224)
Balance, end of year	2,825	3,303	3,546
Allowance for Loan Losses
Individually evaluated for impairment				13	23
Collectively evaluated for impairment				2,645	3,046
Total	3,303	3,546	3,546	2,825	3,303
Loans
Individually evaluated for impairment				2,144	2,588
Collectively evaluated for impairment				306,066	307,600
Loans, net of unearned income				319,183	325,855
Residential real estate \| Receivables Acquired with Deteriorated Credit Quality
Allowance for Loan Losses
Balance, beginning of year	234
Balance, end of year	167	234
Allowance for Loan Losses
Total	234	234		167	234
Loans
Loans, net of unearned income				10,973	15,667
Consumer
Allowance for Loan Losses
Balance, beginning of year	48	60
Charge-offs	(143)	(189)
Recoveries	108	136
Provision	31	41
Balance, end of year	44	48	60
Allowance for Loan Losses
Individually evaluated for impairment				0	0
Collectively evaluated for impairment				44	48
Total	48	60	60	44	48
Loans
Individually evaluated for impairment				5	27
Collectively evaluated for impairment				4,408	4,988
Loans, net of unearned income				4,428	5,031
Consumer \| Receivables Acquired with Deteriorated Credit Quality
Allowance for Loan Losses
Balance, beginning of year	0
Balance, end of year	0	0
Allowance for Loan Losses
Total	0	0		0	0
Loans
Loans, net of unearned income				15	16
Unallocated
Allowance for Loan Losses
Balance, beginning of year	0	0
Charge-offs	0	0
Recoveries	0	0
Provision	0	0
Balance, end of year	0	0	0
Allowance for Loan Losses
Individually evaluated for impairment				0	0
Collectively evaluated for impairment				0	0
Total	0	0	$ 0	0	0
Loans
Individually evaluated for impairment				0	0
Collectively evaluated for impairment				0	0
Loans, net of unearned income				0	0
Unallocated \| Receivables Acquired with Deteriorated Credit Quality
Allowance for Loan Losses
Balance, beginning of year	0
Balance, end of year	0	0
Allowance for Loan Losses
Total	$ 0	$ 0		0	0
Loans
Loans, net of unearned income				$ 0	$ 0