8. LOANS AND LEASES AND ALLOWANCE FOR LOAN AND LEASE LOSSES - Allowance Roll-forward (Details 1) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Allowance for Loan and Lease Losses Beginning balance	$ 4,492	$ 4,881	$ 4,478	$ 4,822
Provision for loan losses	50	300	50	300
Loans charged off	(213)	(673)	(213)	(673)
Recoveries	3	43	17	102
Allowance for Loan and Lease Losses Ending balance	4,332	4,551	4,332	4,551
Allowance related to: Loans individually evaluated for impairment	181		181		$ 355
Allowance related to: Loans collectively evaluated for impairment	4,151		4,151		4,123
Loans	314,802		314,802		313,393
Loans individually evaluated for impairment	9,261		9,261		13,757
Loans collectively evaluated for impairment	305,541		305,541		299,636
Commercial
Allowance for Loan and Lease Losses Beginning balance	669	916	447	855
Provision for loan losses	87	182	300	240
Loans charged off	(213)	(673)	(213)	(673)
Recoveries	1	2	10	5
Allowance for Loan and Lease Losses Ending balance	544	427	544	427
Allowance related to: Loans individually evaluated for impairment	0		0		0
Allowance related to: Loans collectively evaluated for impairment	544		544		447
Loans	24,526		24,526		25,377
Loans individually evaluated for impairment	0		0		1,598
Loans collectively evaluated for impairment	24,526		24,526		23,779
Real estate-commercial
Allowance for Loan and Lease Losses Beginning balance	2,100	2,091	2,174	2,050
Provision for loan losses	(130)	(4)	(208)	(16)
Loans charged off	0	0	0	0
Recoveries	2	1	6	54
Allowance for Loan and Lease Losses Ending balance	1,972	2,088	1,972	2,088
Allowance related to: Loans individually evaluated for impairment	118		118		261
Allowance related to: Loans collectively evaluated for impairment	1,854		1,854		1,913
Loans	190,677		190,677		185,452
Loans individually evaluated for impairment	8,136		8,136		10,070
Loans collectively evaluated for impairment	182,541		182,541		175,382
Real estate-multi-family
Allowance for Loan and Lease Losses Beginning balance	839	789	1,047	851
Provision for loan losses	(99)	209	(307)	147
Loans charged off	0	0	0	0
Recoveries	0	0	0	0
Allowance for Loan and Lease Losses Ending balance	740	998	740	998
Allowance related to: Loans individually evaluated for impairment	0		0		21
Allowance related to: Loans collectively evaluated for impairment	740		740		1,026
Loans	65,312		65,312		78,025
Loans individually evaluated for impairment	0		0		474
Loans collectively evaluated for impairment	65,312		65,312		77,551
Real estate-construction
Allowance for Loan and Lease Losses Beginning balance	298	457	269	446
Provision for loan losses	60	23	89	34
Loans charged off	0	0	0	0
Recoveries	0	0	0	0
Allowance for Loan and Lease Losses Ending balance	358	480	358	480
Allowance related to: Loans individually evaluated for impairment	0		0		0
Allowance related to: Loans collectively evaluated for impairment	358		358		269
Loans	7,486		7,486		5,863
Loans individually evaluated for impairment	0		0		0
Loans collectively evaluated for impairment	7,486		7,486		5,863
Real Estate Residential
Allowance for Loan and Lease Losses Beginning balance	239	268	205	253
Provision for loan losses	1	(37)	35	(22)
Loans charged off	0	0	0	0
Recoveries	0	0	0	0
Allowance for Loan and Lease Losses Ending balance	240	231	240	231
Allowance related to: Loans individually evaluated for impairment	63		63		73
Allowance related to: Loans collectively evaluated for impairment	177		177		132
Loans	16,697		16,697		15,813
Loans individually evaluated for impairment	1,057		1,057		1,615
Loans collectively evaluated for impairment	15,640		15,640		14,198
Leases
Allowance for Loan and Lease Losses Beginning balance	0	1	0	1
Provision for loan losses	0	(40)	(1)	(40)
Loans charged off	0	0	0	0
Recoveries	0	39	1	39
Allowance for Loan and Lease Losses Ending balance	0	0	0	0
Allowance related to: Loans individually evaluated for impairment	0		0		0
Allowance related to: Loans collectively evaluated for impairment	0		0		0
Loans	61		61		205
Loans individually evaluated for impairment	0		0		0
Loans collectively evaluated for impairment	61		61		205
Agriculture
Allowance for Loan and Lease Losses Beginning balance	49	59	31	64
Provision for loan losses	46	(30)	64	(35)
Loans charged off	0	0	0	0
Recoveries	0	0	0	0
Allowance for Loan and Lease Losses Ending balance	95	29	95	29
Allowance related to: Loans individually evaluated for impairment	0		0		0
Allowance related to: Loans collectively evaluated for impairment	95		95		31
Loans	4,591		4,591		1,713
Loans individually evaluated for impairment	0		0		0
Loans collectively evaluated for impairment	4,591		4,591		1,713
Consumer
Allowance for Loan and Lease Losses Beginning balance	11	19	14	24
Provision for loan losses	83	(3)	80	(11)
Loans charged off	0	0	0	0
Recoveries	0	1	0	4
Allowance for Loan and Lease Losses Ending balance	94	17	94	17
Allowance related to: Loans individually evaluated for impairment	0		0		0
Allowance related to: Loans collectively evaluated for impairment	94		94		14
Loans	5,452		5,452		945
Loans individually evaluated for impairment	68		68		0
Loans collectively evaluated for impairment	5,384		5,384		945
Unallocated
Allowance for Loan and Lease Losses Beginning balance	287	281	291	278
Provision for loan losses	2	0	(2)	3
Loans charged off	0	0	0	0
Recoveries	0	0	0	0
Allowance for Loan and Lease Losses Ending balance	289	$ 281	289	$ 281
Allowance related to: Loans individually evaluated for impairment	0		0		0
Allowance related to: Loans collectively evaluated for impairment	289		289		291
Loans	0		0		0
Loans individually evaluated for impairment	0		0		0
Loans collectively evaluated for impairment	$ 0		$ 0		$ 0

A valuation allowance for financing receivables that are expected to be uncollectible.

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

- 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

Amount of direct write-downs of financing receivables charged against the allowance.

+ References

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesWriteOffs
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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 before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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

+ Details

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

- Definition

Amount of expense related loan transactions, lease transactions, credit loss from transactions other than loan and lease transactions, and other loss based on assessment of uncollectability from the counterparty to reduce the account to their net realizable value.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-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

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.11)
-URI http://asc.fasb.org/extlink&oid=114873765&loc=SL114874048-224260

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3602-108585

+ Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=AMRB_TotalCommercialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=AMRB_RealEstateMultiFamilyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=AMRB_RealEstateConstructionMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=AMRB_TotalResidentialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_LeaseAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=AMRB_AgricultureMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=AMRB_TotalConsumerMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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