Allowance For Loan Losses - Recorded Investment Segregated By Amounts Individually Or Collectively In Allowance For Loan Losses (Details) - USD ($) $ in Thousands	Dec. 31, 2019	Dec. 31, 2018
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	$ 1,043,371	$ 984,346
Individually evaluated for impairment	24,673	31,714
Collectively evaluated for impairment	1,018,698	952,632
Commercial
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	411,327	341,091
Individually evaluated for impairment	16,077	24,189
Collectively evaluated for impairment	395,250	316,902
Real estate construction one-to-four family
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	38,503	36,828
Individually evaluated for impairment	1,349	0
Collectively evaluated for impairment	37,154	36,828
Real estate construction other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	60,906	71,658
Individually evaluated for impairment	0	0
Collectively evaluated for impairment	60,906	71,658
Real estate term owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	138,181	125,795
Individually evaluated for impairment	5,104	5,253
Collectively evaluated for impairment	133,077	120,542
Real estate term non-owner occupied
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	311,302	324,198
Individually evaluated for impairment	178	760
Collectively evaluated for impairment	311,124	323,438
Real estate term other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	42,200	41,746
Individually evaluated for impairment	1,594	1,063
Collectively evaluated for impairment	40,606	40,683
Consumer secured by 1st deeds of trust
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	16,191	19,234
Individually evaluated for impairment	281	449
Collectively evaluated for impairment	15,910	18,785
Consumer other
Financing Receivable, Allowance for Credit Loss [Line Items]
Total loans	24,761	23,796
Individually evaluated for impairment	90	0
Collectively evaluated for impairment	$ 24,671	$ 23,796

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.

No definition available.

+ Details

Name:	us-gaap_FinancingReceivableAllowanceForCreditLossesLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
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

+ 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=118952595&loc=d3e4428-111522

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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