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Loans - Troubled Debt Restructurings (Details) (USD $)
3 Months Ended 12 Months Ended
Mar. 31, 2015
Contract
Mar. 31, 2014
Contract
Dec. 31, 2014
Contract
Financing Receivable, Modifications [Line Items]      
TDR that Subsequently Defaulted, Number of Contracts 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Financing Receivable, Modifications, Subsequent Default, Recorded Investment $ 0us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1 $ 353,000us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1  
Number of Contracts 2us-gaap_FinancingReceivableModificationsNumberOfContracts2    
Pre-Modification Recorded Investment 900,000us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2    
Post-Modification Recorded Investment 1,354,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2    
Existing Troubled Debt Restructuring 7,679,000nrim_FinancingReceivableModificationsinDifferentPeriodRecordedInvestment    
Loans classified as troubled debt restructuring 9,033,000us-gaap_FinancingReceivableModificationsRecordedInvestment   7,700,000us-gaap_FinancingReceivableModificationsRecordedInvestment
Rate Modification      
Financing Receivable, Modifications [Line Items]      
Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_RateModificationMember
   
Post-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_RateModificationMember
   
Term Modification      
Financing Receivable, Modifications [Line Items]      
Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_TermModificationMember
   
Post-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_TermModificationMember
   
Payment Modification      
Financing Receivable, Modifications [Line Items]      
Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_PaymentModificationMember
   
Post-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_PaymentModificationMember
   
Combination Modification      
Financing Receivable, Modifications [Line Items]      
Pre-Modification Recorded Investment 900,000us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_CombinationModificationMember
   
Post-Modification Recorded Investment 1,354,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_CombinationModificationMember
   
Commercial | Asset Quality Rating - Special Mention      
Financing Receivable, Modifications [Line Items]      
Number of Contracts 2us-gaap_FinancingReceivableModificationsNumberOfContracts2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
   
Pre-Modification Recorded Investment 900,000us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
   
Post-Modification Recorded Investment 1,354,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
   
Commercial | Asset Quality Rating - Special Mention | Rate Modification      
Financing Receivable, Modifications [Line Items]      
Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_RateModificationMember
   
Post-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_RateModificationMember
   
Commercial | Asset Quality Rating - Special Mention | Term Modification      
Financing Receivable, Modifications [Line Items]      
Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_TermModificationMember
   
Post-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_TermModificationMember
   
Commercial | Asset Quality Rating - Special Mention | Payment Modification      
Financing Receivable, Modifications [Line Items]      
Pre-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_PaymentModificationMember
   
Post-Modification Recorded Investment 0us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_PaymentModificationMember
   
Commercial | Asset Quality Rating - Special Mention | Combination Modification      
Financing Receivable, Modifications [Line Items]      
Pre-Modification Recorded Investment 900,000us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_CombinationModificationMember
   
Post-Modification Recorded Investment 1,354,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ nrim_TroubledDebtRestructuringModificationTypeAxis
= nrim_CombinationModificationMember
   
Real Estate Construction One-To- Four Family | Asset Quality Rating - Special Mention      
Financing Receivable, Modifications [Line Items]      
TDR that Subsequently Defaulted, Number of Contracts 0us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nrim_RealEstateConstructionOneToFourFamilyPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
1us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nrim_RealEstateConstructionOneToFourFamilyPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
 
Financing Receivable, Modifications, Subsequent Default, Recorded Investment 0us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nrim_RealEstateConstructionOneToFourFamilyPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
353,000us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= nrim_RealEstateConstructionOneToFourFamilyPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
 
Loans with Accrual Status      
Financing Receivable, Modifications [Line Items]      
Post-Modification Recorded Investment 150,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ nrim_LoansByAccrualStatusAxis
= nrim_LoanswithAccrualStatusMember
   
Existing Troubled Debt Restructuring 5,638,000nrim_FinancingReceivableModificationsinDifferentPeriodRecordedInvestment
/ nrim_LoansByAccrualStatusAxis
= nrim_LoanswithAccrualStatusMember
   
Loans classified as troubled debt restructuring 5,788,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ nrim_LoansByAccrualStatusAxis
= nrim_LoanswithAccrualStatusMember
   
Loans with Accrual Status | Commercial | Asset Quality Rating - Special Mention      
Financing Receivable, Modifications [Line Items]      
Post-Modification Recorded Investment 150,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ nrim_LoansByAccrualStatusAxis
= nrim_LoanswithAccrualStatusMember
   
Loans with Non-Accrual Status      
Financing Receivable, Modifications [Line Items]      
Post-Modification Recorded Investment 1,204,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ nrim_LoansByAccrualStatusAxis
= nrim_LoanswithNonAccrualStatusMember
   
Existing Troubled Debt Restructuring 2,041,000nrim_FinancingReceivableModificationsinDifferentPeriodRecordedInvestment
/ nrim_LoansByAccrualStatusAxis
= nrim_LoanswithNonAccrualStatusMember
   
Loans classified as troubled debt restructuring 3,245,000us-gaap_FinancingReceivableModificationsRecordedInvestment
/ nrim_LoansByAccrualStatusAxis
= nrim_LoanswithNonAccrualStatusMember
   
Loans with Non-Accrual Status | Commercial | Asset Quality Rating - Special Mention      
Financing Receivable, Modifications [Line Items]      
Post-Modification Recorded Investment $ 1,204,000us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
/ us-gaap_FinancingReceivableTroubledDebtRestructuringAxis
= us-gaap_CommercialPortfolioSegmentMember
/ us-gaap_InternalCreditAssessmentAxis
= us-gaap_SpecialMentionMember
/ nrim_LoansByAccrualStatusAxis
= nrim_LoanswithNonAccrualStatusMember