Loans and Allowance for Credit Losses - Loan Modifications not Classified as TDRs (Details) $ in Thousands	3 Months Ended
Mar. 31, 2021 USD ($) contract
Receivables [Abstract]
Portfolio loans: Interest Only \| $	$ 65,201
Portfolio loans: Full Payment Deferral \| $	23,096
Portfolio loans: Total \| $	$ 88,297
Number of modifications: Interest Only \| contract	21,000
Number of modifications: Full Payment Deferral \| contract	9,000
Number of modifications: Total \| contract	30,000

Financing Receivable, Modifications, Non Troubled Debt, Full Payment Deferral

No definition available.

Name:	nrim_FinancingReceivableModificationsNonTroubledDebtFullPaymentDeferral
Namespace Prefix:	nrim_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Financing Receivable, Modifications, Non Troubled Debt, Interest

No definition available.

Name:	nrim_FinancingReceivableModificationsNonTroubledDebtInterest
Namespace Prefix:	nrim_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Financing Receivables, Modifications, Non Troubled Debt, Amount

No definition available.

Name:	nrim_FinancingReceivablesModificationsNonTroubledDebtAmount
Namespace Prefix:	nrim_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Financing Receivables, Modifications, Non Troubled Debt, Full Payment Deferral

No definition available.

Name:	nrim_FinancingReceivablesModificationsNonTroubledDebtFullPaymentDeferral
Namespace Prefix:	nrim_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Financing Receivables, Modifications, Non Troubled Debt, Interest, Number Of Contracts

No definition available.

Name:	nrim_FinancingReceivablesModificationsNonTroubledDebtInterestNumberOfContracts
Namespace Prefix:	nrim_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Financing Receivables, Modifications, Non-Troubled Debt, Number Of Contracts

No definition available.

Name:	nrim_FinancingReceivablesModificationsNonTroubledDebtNumberOfContracts
Namespace Prefix:	nrim_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

No definition available.

Name:	us-gaap_ReceivablesAbstract
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration