Equity Securities Sold During Period

No definition available.

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

- Definition

This line item represents federal home loan bank letters of credit which were used as additional collateral amount.

+ References

No definition available.

+ Details

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

- Definition

Represents the percentage of par value of restricted stock out of outstanding borrowings required by Federal Home Loan Bank.

+ References

No definition available.

+ Details

Name:	amnb_ParValueofRestrictedStockOutofOutstandingBorrowingsRequiredbyFHLB
Namespace Prefix:	amnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Represents the percentage of par value of restricted stock out of outstanding capital required by Federal Reserve Bank.

+ References

No definition available.

+ Details

Name:	amnb_ParValueofRestrictedStockOutofOutstandingCapitalRequiredbyFRB
Namespace Prefix:	amnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of Par Value of Federal Reserve Banks Restricted Stock Outstanding Capital Subject to Call

+ References

No definition available.

+ Details

Name:	amnb_PercentageofParValueofFederalReserveBanksRestrictedStockOutstandingCapitalSubjecttoCall
Namespace Prefix:	amnb_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for 12 months or longer, without an allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922888-210455

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of restricted investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 860
-SubTopic 30
-Section 50
-Paragraph 1A
-Subparagraph (b)(1)(i)
-URI http://asc.fasb.org/extlink&oid=109249958&loc=SL6224234-111729

+ Details

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

- Definition

Number of investments in debt securities measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position, without allowance for credit loss. Includes beneficial interest in securitized financial asset.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 6
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=118939692&loc=d3e27290-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (b)(3)
-URI http://asc.fasb.org/extlink&oid=118955579&loc=SL82922888-210455

+ Details

Name:	us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionNumberOfPositions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Amount of unrealized gain on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 321
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=116654751&loc=SL75117546-209714

+ Details

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

- Definition

Federal Home Loan Bank (FHLB) stock represents an equity interest in a FHLB. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 325
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=75031641&loc=d3e63345-112809

+ Details

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

- Definition

Federal Reserve Bank stock represents an equity interest in the Federal Reserve Bank. It does not have a readily determinable fair value because its ownership is restricted and it lacks a market (liquidity).

+ References

+ Details

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

- Definition

The amount by which the fair value of an investment is less than the amortized cost basis or carrying amount of that investment at the balance sheet date and the decline in fair value is deemed to be other than temporary, before considering whether or not such amount is recognized in earnings or other comprehensive income.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 8A
-URI http://asc.fasb.org/extlink&oid=118954083&loc=SL6284422-111562

+ Details

Name:	us-gaap_ImpairmentOfInvestments
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_ScheduleOfAvailableForSaleSecuritiesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_FinancialInstrumentAxis=amnb_FederalAgenciesandGSEsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_ResidentialMortgageBackedSecuritiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_CollateralizedMortgageObligationsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_USStatesAndPoliticalSubdivisionsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Securities - Narrative (Details)	12 Months Ended
Securities - Narrative (Details)	Dec. 31, 2019 USD ($) security	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Debt Securities, Available-for-sale [Line Items]
Net unrealized holding gain on equity securities	$ 333,000	$ 42,000	$ 0
Equity securities sold during period	445,000
Securities pledged to secure public deposits, repurchase agreements and other	179,852,000	143,064,000
FHLB letters of credit used as additional collateral amount	$ 170,000,000	190,250,000
Federal Home Loan Bank stock requirement, percentage of outstanding borrowings	4.25%
Federal Reserve Bank stock, stock requirement, percentage of outstanding capital	3.00%
Federal Reserve Bank stock, stock requirement, percentage subject to call	3.00%
Federal reserve bank stock	$ 6,415,000	3,621,000
Investment in FHLB stock	2,215,000	1,626,000
Other-than-temporary impairment losses recognized	$ 0	$ 0
Federal agencies and GSEs
Debt Securities, Available-for-sale [Line Items]
Debt securities, available for sale, unrealized loss position, number of positions \| security	26
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Debt securities, available for sale, unrealized loss position, number of positions \| security	32
Debt securities, available for sale, continuous unrealized loss positions, 12 months or longer, number of positions \| security	15
Collateralized mortgage obligations
Debt Securities, Available-for-sale [Line Items]
Debt securities, available for sale, unrealized loss position, number of positions \| security	1
Debt securities, available for sale, continuous unrealized loss positions, 12 months or longer, number of positions \| security	1
State and municipal
Debt Securities, Available-for-sale [Line Items]
Debt securities, available for sale, unrealized loss position, number of positions \| security	6