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Loans Held For Sale (Details) (USD $)
In Thousands, unless otherwise specified
 12 Months Ended 1 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Dec. 31, 2014
Dec. 02, 2014
Nov. 30, 2014
Business Acquisition [Line Items]            
Payments for Origination of Mortgage Loans Held-for-sale $ 62,652us-gaap_PaymentsForOriginationOfMortgageLoansHeldForSale $ 0us-gaap_PaymentsForOriginationOfMortgageLoansHeldForSale $ 0us-gaap_PaymentsForOriginationOfMortgageLoansHeldForSale      
Payments to Purchase Loans Held-for-sale 132,013us-gaap_PaymentsToPurchaseLoansHeldForSale 156,521us-gaap_PaymentsToPurchaseLoansHeldForSale 242,535us-gaap_PaymentsToPurchaseLoansHeldForSale      
Proceeds from the sale of loans held for sale 203,404us-gaap_ProceedsFromSaleOfLoansHeldForSale 156,925us-gaap_ProceedsFromSaleOfLoansHeldForSale 258,652us-gaap_ProceedsFromSaleOfLoansHeldForSale      
RML            
Business Acquisition [Line Items]            
Voting interests acquired         76.50%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember		  
Equity interest percentage         100.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember		 23.50%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
RML            
Business Acquisition [Line Items]            
Payments for Origination of Mortgage Loans Held-for-sale       $ 62,700us-gaap_PaymentsForOriginationOfMortgageLoansHeldForSale
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember