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Loans - Narrative (Details) (USD $)
3 Months Ended 12 Months Ended 0 Months Ended
Mar. 31, 2015
Contract
financing_commitment
Mar. 31, 2014
Contract
Dec. 31, 2014
Contract
Apr. 01, 2014
purchased_loan
Apr. 02, 2014
Business Acquisition [Line Items]          
Financing Receivable, Modifications, Specific Contract, Allowance for Credit Losses $ 215,000nrim_FinancingReceivableModificationsSpecificContractAllowanceforCreditLosses        
Nonaccrual loans 4,810,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus   4,674,000us-gaap_FinancingReceivableRecordedInvestmentNonaccrualStatus    
Nonperforming loans minimum threshold for individual impairment evaluation 50,000nrim_NonperformingLoansMinimumThresholdForIndividualImpairmentEvaluation        
Recorded investment in loans considered to be impaired 12,827,000us-gaap_ImpairedFinancingReceivableRecordedInvestment   11,297,000us-gaap_ImpairedFinancingReceivableRecordedInvestment    
Fair value of purchased credit impaired loans 2,800,000us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAtFairValue       3,900,000us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAtFairValue
Loans classified as troubled debt restructuring 9,033,000us-gaap_FinancingReceivableModificationsRecordedInvestment   7,700,000us-gaap_FinancingReceivableModificationsRecordedInvestment    
Number of commitments on TDR 0nrim_FinancingReceivableModificationsNumberofCommitmentstoExtendFinancing        
TDR Write-downs 0us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown        
TDR with specific allowance, Number of Contracts 2nrim_FinancingReceivableModificationswithSpecificAllowanceNumberofContracts   0nrim_FinancingReceivableModificationswithSpecificAllowanceNumberofContracts    
Post-Modification Recorded Investment 1,354,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2        
TDR that Subsequently Defaulted, Number of Contracts 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1    
Alaska Pacific Bank          
Business Acquisition [Line Items]          
Purchased impaired loans       18nrim_FinancingReceivableAcquiredwithDeterioratedCreditQualityNumberofLoans
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
 
Accretable difference on purchased loans 0us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAccretableYield
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
       
Fair value of purchased credit impaired loans         $ 3,899,000us-gaap_CertainLoansAcquiredInTransferNotAccountedForAsDebtSecuritiesAcquiredDuringPeriodAtAcquisitionAtFairValue
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember