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Derivatives - Narrative (Details) (USD $)
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 31, 2014
Derivatives, Fair Value [Line Items]      
Collateral requirement threshold $ 250,000nrim_DerivativesValueShortfallsThresholdTriggeringAdditionalCollateralRequirements    
Collateral posted 262,000us-gaap_CollateralAlreadyPostedAggregateFairValue   0us-gaap_CollateralAlreadyPostedAggregateFairValue
Derivative notional amount 23,000,000invest_DerivativeNotionalAmount   23,600,000invest_DerivativeNotionalAmount
Derivative fee income 0us-gaap_BankingFeesAndCommissions 0us-gaap_BankingFeesAndCommissions  
Variable to Fixed      
Derivatives, Fair Value [Line Items]      
Derivative notional amount 11,500,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= nrim_VariabletoFixedMember
   
Number of derivatives 2us-gaap_DerivativeNumberOfInstrumentsHeld
/ us-gaap_DerivativeInstrumentRiskAxis
= nrim_VariabletoFixedMember
   
Fixed to Variable      
Derivatives, Fair Value [Line Items]      
Derivative notional amount 11,500,000invest_DerivativeNotionalAmount
/ us-gaap_DerivativeInstrumentRiskAxis
= nrim_FixedtoVariableMember
   
Number of derivatives 2us-gaap_DerivativeNumberOfInstrumentsHeld
/ us-gaap_DerivativeInstrumentRiskAxis
= nrim_FixedtoVariableMember
   
Residential Mortgage Holding Company      
Derivatives, Fair Value [Line Items]      
Unfunded Lending Commitment $ 81,200,000nrim_UnfundedLendingCommitment
/ dei_LegalEntityAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
  $ 39,600,000nrim_UnfundedLendingCommitment
/ dei_LegalEntityAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember