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Loans Held for Sale (Details) (USD $)
In Thousands, unless otherwise specified
3 Months Ended
Mar. 31, 2015
Mar. 31, 2014
Dec. 02, 2014
Nov. 30, 2014
Business Acquisition [Line Items]        
Payments for Origination of Mortgage Loans Held-for-sale $ 176,372us-gaap_PaymentsForOriginationOfMortgageLoansHeldForSale $ 0us-gaap_PaymentsForOriginationOfMortgageLoansHeldForSale    
Purchases of loans held for sale 0us-gaap_PaymentsToPurchaseLoansHeldForSale 30,482us-gaap_PaymentsToPurchaseLoansHeldForSale    
Proceeds from the sale of loans held for sale 158,365us-gaap_ProceedsFromSaleOfLoansHeldForSale 27,455us-gaap_ProceedsFromSaleOfLoansHeldForSale    
Residential Mortgage Holding Company        
Business Acquisition [Line Items]        
Voting interests acquired     76.50%us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
 
Ownership Percentage     100.00%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
23.50%us-gaap_EquityMethodInvestmentOwnershipPercentage
/ us-gaap_BusinessAcquisitionAxis
= nrim_ResidentialMortgageHoldingCompanyLlcMember
Subsidiaries        
Business Acquisition [Line Items]        
Payments for Origination of Mortgage Loans Held-for-sale 176,400us-gaap_PaymentsForOriginationOfMortgageLoansHeldForSale
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember
     
Proceeds from the sale of loans held for sale $ 158,400us-gaap_ProceedsFromSaleOfLoansHeldForSale
/ dei_LegalEntityAxis
= us-gaap_SubsidiariesMember