Allowance for Loan Losses - Recorded Investment Segregated By Amounts Individually Or Collectively In Allowance For Loan Losses (Details) - USD ($) $ in Thousands	Sep. 30, 2018	Dec. 31, 2017
Financing Receivable, Allowance for Credit Losses [Line Items]
Total financing receivables	$ 986,362	$ 959,109
Balance, end of period: Individually evaluated for impairment	35,434	31,967
Balance, end of period: Collectively evaluated for impairment	950,928	927,142
Commercial
Financing Receivable, Allowance for Credit Losses [Line Items]
Total financing receivables	333,132	313,514
Balance, end of period: Individually evaluated for impairment	27,167	26,812
Balance, end of period: Collectively evaluated for impairment	305,965	286,702
Real estate construction one-to-four family
Financing Receivable, Allowance for Credit Losses [Line Items]
Total financing receivables	36,119	31,201
Balance, end of period: Individually evaluated for impairment	0	0
Balance, end of period: Collectively evaluated for impairment	36,119	31,201
Real estate construction other
Financing Receivable, Allowance for Credit Losses [Line Items]
Total financing receivables	61,857	80,093
Balance, end of period: Individually evaluated for impairment	0	0
Balance, end of period: Collectively evaluated for impairment	61,857	80,093
Real estate term owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Total financing receivables	130,166	132,042
Balance, end of period: Individually evaluated for impairment	5,789	2,862
Balance, end of period: Collectively evaluated for impairment	124,377	129,180
Real estate term non-owner occupied
Financing Receivable, Allowance for Credit Losses [Line Items]
Total financing receivables	340,820	319,313
Balance, end of period: Individually evaluated for impairment	788	874
Balance, end of period: Collectively evaluated for impairment	340,032	318,439
Real estate term other
Financing Receivable, Allowance for Credit Losses [Line Items]
Total financing receivables	41,493	40,411
Balance, end of period: Individually evaluated for impairment	1,081	559
Balance, end of period: Collectively evaluated for impairment	40,412	39,852
Consumer secured by 1st deeds of trust
Financing Receivable, Allowance for Credit Losses [Line Items]
Total financing receivables	20,202	22,616
Balance, end of period: Individually evaluated for impairment	609	860
Balance, end of period: Collectively evaluated for impairment	19,593	21,756
Consumer other
Financing Receivable, Allowance for Credit Losses [Line Items]
Total financing receivables	22,573	19,919
Balance, end of period: Individually evaluated for impairment	0	0
Balance, end of period: Collectively evaluated for impairment	$ 22,573	$ 19,919

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=84173941&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

Net amount of the investment in a contractual right to receive money on demand or on fixed or determinable dates that is recognized as an asset in the creditor's statement of financial position. Examples include, but are not limited to, credit card receivables, notes receivable and receivables relating to lessor's rights to payments from leases other than operating leases that have been recorded as assets. Excludes trade accounts receivable with contractual maturity of one year or less and arose from the sale of goods or services.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.3)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

Name:	us-gaap_NotesReceivableNet
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: