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Business Combinations - Summary of Fair Value Adjustments (Details) (USD $)
In Thousands, unless otherwise specified
0 Months Ended
Apr. 01, 2014
Dec. 02, 2014
Mar. 31, 2015
Dec. 31, 2014
Apr. 02, 2014
Business Acquisition [Line Items]          
Goodwill     $ (22,334)us-gaap_Goodwill $ (22,334)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
       
Bargain purchase gain 170us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount
/ us-gaap_BusinessAcquisitionAxis
= nrim_AlaskaPacificBankMember
       
Residential Mortgage Holding Company          
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
     
Net loans   (360)nrim_BusinessCombinationRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedLoansFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
     
Other intangible assets   950nrim_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedIntangibleAssetsOtherthanGoodwillFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
     
Other   (311)nrim_BusinessCombinationRecognizedIdentifiableAssetsAcquiredandLiabilitiesAssumedNoncurrentLiabilitiesOtherFairValueAdjustment
/ 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   (7,318)us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
(8,800)us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
(7,300)us-gaap_BusinessCombinationContingentConsiderationLiability
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Rate lock derivative asset   960nrim_BusinessCombinationRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDerivativeAssetFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
     
Investment in Homestate Mortgage, LLC   1,490nrim_BusinessCombinationRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedInvestmentFairValueAdjustment
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
     
Goodwill   $ (14,810)us-gaap_Goodwill
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember