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Business Combinations - Summary of Fair Value Adjustments (Details) (USD $)
In Thousands, unless otherwise specified
12 Months Ended 0 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Apr. 01, 2014
Dec. 02, 2014
Apr. 02, 2014
Business Acquisition [Line Items]            
Gain on purchase of mortgage affiliate $ 3,001us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount $ 0us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount $ 0us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount      
Goodwill (22,334)us-gaap_Goodwill (7,525)us-gaap_Goodwill        
Alaska Pacific Bank            
Business Acquisition [Line Items]            
Cost basis net assets           14,733nrim_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillandLiabilitiesAssumedCostBasisNet
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
Cash payment made       (6,423)us-gaap_PaymentsToAcquireBusinessesGross
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
   
Common stock issued       (7,446)us-gaap_BusinessCombinationConsiderationTransferredEquityInterestsIssuedAndIssuable
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
   
Net loans       (1,137)nrim_BusinessCombinationRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedLoansFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
   
Premises and equipment       547nrim_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedPropertyPlantandEquipmentFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
   
Other intangible assets       623nrim_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedIntangibleAssetsOtherthanGoodwillFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
   
Mortgage servicing rights       (119)nrim_BusinessCombinationRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedMortgageServicingRightsFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
   
Deposits       (844)nrim_BusinessCombinationRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDepositsFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
   
Other       236nrim_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentLiabilitiesOtherFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
   
Gain on purchase of mortgage affiliate       170us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
   
RML            
Business Acquisition [Line Items]            
Cost basis net assets         11,915nrim_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillandLiabilitiesAssumedCostBasisNet
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Cash payment made         (18,240)us-gaap_PaymentsToAcquireBusinessesGross
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Cash surrender value of life insurance paid         (3,896)nrim_BusinessCombinationRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedCashSurrenderValueofLifeInsurancePaidFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Liability for future earn out payments (7,300)us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
      (7,318)us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Net loans         (360)nrim_BusinessCombinationRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedLoansFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Other intangible assets         950nrim_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedIntangibleAssetsOtherthanGoodwillFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Rate lock derivative asset         960nrim_BusinessCombinationRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDerivativeAssetFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Investment in Homestate         1,490nrim_BusinessCombinationRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedInvestmentFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Other         (311)nrim_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentLiabilitiesOtherFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Goodwill         $ (14,810)us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember