Reserve for loans impairments recorded after acquisition as a result of current information and events which result in it being probable that estimated cash flows will be less than previously expected or because actual cash flows are less than cash flows previously expected. This relates to loans not accounted for as debt securities, with evidence of deterioration of credit quality since origination that was acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118938799&loc=d3e13503-111538

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(2)
-URI http://asc.fasb.org/extlink&oid=118939070&loc=d3e15032-111544

+ Details

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

- Definition

The recorded balance, net of any applicable reductions (for example, unamortized accretable yield and allowance for loan losses). This relates to loans not accounted for debt securities with evidence of deterioration of credit quality since origination that were acquired by completion of a transfer for which it is probable, at acquisition, that the investor will be unable to collect all contractually required payments receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (a)(1)
-URI http://asc.fasb.org/extlink&oid=118939070&loc=d3e15032-111544

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 15
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118938799&loc=d3e13503-111538

+ Details

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

- 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

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 (g)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

+ 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

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=118955255&loc=SL82922352-210448

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-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 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (e)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(4)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

+ Details

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

- Definition

Amount of writeoff of financing receivable, charged against allowance for credit loss.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 13
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919249-210447

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(3)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 3: http://www.xbrl.org/2003/role/exampleRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 55
-Paragraph 79
-URI http://asc.fasb.org/extlink&oid=118955255&loc=SL82922352-210448

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-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

+ 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

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 (h)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

+ 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

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 (h)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

+ 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-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=118939070&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=118936363&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-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

+ Details

Name:	us-gaap_LoansAndLeasesReceivableAllowance
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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=118952595&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 to estimated loss from loan and lease transactions.

+ References

Reference 1: 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=118932676&loc=d3e3602-108585

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=116637391&loc=SL114874048-224260

Reference 3: 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=118936363&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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=banr_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:

- Details

Name:	banr_CumulativeEffectPeriodOfAdoptionAxis=banr_CumulativeEffectPeriodOfAdoptionAdjustmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

LOANS RECEIVABLE AND THE ALLOWANCE FOR LOAN LOSSES (Allowance for Loan Losses) (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Mar. 31, 2020	Jan. 01, 2020	Dec. 31, 2019	Mar. 31, 2019
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	$ 100,559	$ 96,485
Provision for credit losses	21,713	2,000
Recoveries	2,366	219
Charge-offs	(1,962)	(1,396)
Allowance for loan losses, Ending balance	130,488	97,308
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment						$ 385
Allowance collectively evaluated for impairment						96,842
Allowance for purchased credit-impaired loans						81
Total allowance for loan losses	130,488	97,308	$ 130,488		$ 100,559	97,308
Loans individually evaluated for impairment						21,727
Loans collectively evaluated for impairment						8,657,600
Purchased Credit-Impaired					15,938	13,330
Total loans			9,285,744		9,305,357	8,692,657
Commercial real estate
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	30,591	27,132
Provision for credit losses	1,545	369
Recoveries	167	21
Charge-offs	(100)	(431)
Allowance for loan losses, Ending balance	29,339	27,091
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment						240
Allowance collectively evaluated for impairment						26,851
Allowance for purchased credit-impaired loans						0
Total allowance for loan losses	29,339	27,091	29,339		30,591	27,091
Loans individually evaluated for impairment						9,806
Loans collectively evaluated for impairment						3,545,162
Purchased Credit-Impaired						11,805
Total loans						3,566,773
Multifamily real estate
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	4,754	3,818
Provision for credit losses	321	202
Recoveries	0	0
Charge-offs	(66)	0
Allowance for loan losses, Ending balance	2,805	4,020
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment						0
Allowance collectively evaluated for impairment						4,020
Allowance for purchased credit-impaired loans						0
Total allowance for loan losses	2,805	4,020	2,805		4,754	4,020
Loans individually evaluated for impairment						0
Loans collectively evaluated for impairment						387,014
Purchased Credit-Impaired					7	128
Total loans			400,206		473,152	387,142
Construction and land
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	22,994	24,442
Provision for credit losses	8,708	(751)
Recoveries	0	22
Charge-offs	0	0
Allowance for loan losses, Ending balance	34,217	23,713
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment						0
Allowance collectively evaluated for impairment						23,713
Allowance for purchased credit-impaired loans						0
Total allowance for loan losses	34,217	23,713	34,217		22,994	23,713
Loans individually evaluated for impairment						2,988
Loans collectively evaluated for impairment						1,093,159
Purchased Credit-Impaired						0
Total loans						1,096,147
Commercial business
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	23,370	19,438
Provision for credit losses	6,447	(209)
Recoveries	205	23
Charge-offs	(1,384)	(590)
Allowance for loan losses, Ending balance	31,648	18,662
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment						12
Allowance collectively evaluated for impairment						18,627
Allowance for purchased credit-impaired loans						23
Total allowance for loan losses	31,648	18,662	31,648		23,370	18,662
Loans individually evaluated for impairment						514
Loans collectively evaluated for impairment						1,523,166
Purchased Credit-Impaired					368	618
Total loans			1,357,817		1,693,824	1,524,298
Agricultural business
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	4,120	3,778
Provision for credit losses	(1,006)	(178)
Recoveries	1,750	0
Charge-offs	0	(4)
Allowance for loan losses, Ending balance	4,513	3,596
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment						66
Allowance collectively evaluated for impairment						3,472
Allowance for purchased credit-impaired loans						58
Total allowance for loan losses	4,513	3,596	4,513		4,120	3,596
Loans individually evaluated for impairment						4,110
Loans collectively evaluated for impairment						368,781
Purchased Credit-Impaired					393	431
Total loans			330,257		370,549	373,322
One-to four-family residential
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	4,136	4,714
Provision for credit losses	539	(46)
Recoveries	148	43
Charge-offs	(64)	0
Allowance for loan losses, Ending balance	11,884	4,711
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment						59
Allowance collectively evaluated for impairment						4,652
Allowance for purchased credit-impaired loans						0
Total allowance for loan losses	11,884	4,711	11,884		4,136	4,711
Loans individually evaluated for impairment						4,116
Loans collectively evaluated for impairment						963,370
Purchased Credit-Impaired					74	95
Total loans			881,387		945,622	967,581
Consumer [Member]
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	8,202	7,972
Provision for credit losses	5,159	269
Recoveries	96	110
Charge-offs	(348)	(371)
Allowance for loan losses, Ending balance	16,082	7,980
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment						8
Allowance collectively evaluated for impairment						7,972
Allowance for purchased credit-impaired loans						0
Total allowance for loan losses	16,082	7,980	16,082		8,202	7,980
Loans individually evaluated for impairment						193
Loans collectively evaluated for impairment						776,948
Purchased Credit-Impaired						253
Total loans						777,394
Unallocated
Financing Receivable, Allowance for Loan Losses [Roll Forward]
Allowance for loan losses, Beginning balance	2,392	5,191
Provision for credit losses	0	2,344
Recoveries	0	0
Charge-offs	0	0
Allowance for loan losses, Ending balance	0	7,535
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Allowance individually evaluated for impairment						0
Allowance collectively evaluated for impairment						7,535
Allowance for purchased credit-impaired loans						0
Total allowance for loan losses	$ 0	$ 7,535	$ 0		$ 2,392	7,535
Loans individually evaluated for impairment						0
Loans collectively evaluated for impairment						0
Purchased Credit-Impaired						0
Total loans						$ 0
Impact of Topic 326 Adoption
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses				$ 7,812
Impact of Topic 326 Adoption \| Commercial real estate
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses				(2,864)
Impact of Topic 326 Adoption \| Multifamily real estate
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses				(2,204)
Impact of Topic 326 Adoption \| Construction and land
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses				2,515
Impact of Topic 326 Adoption \| Commercial business
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses				3,010
Impact of Topic 326 Adoption \| Agricultural business
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses				(351)
Impact of Topic 326 Adoption \| One-to four-family residential
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses				7,125
Impact of Topic 326 Adoption \| Consumer [Member]
Financing Receivable, Allowance for Loan Losses, Additional Information [Abstract]
Total allowance for loan losses				$ 2,973