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Deposits - Narrative (Details) (USD $)
Dec. 31, 2014
Dec. 31, 2013
Deposits [Line Items]    
Certificates of deposit $ 142,212,000us-gaap_InterestBearingDomesticDepositCertificatesOfDeposits  
Certificates of Deposit from Public Entities Collateralized by Letters of Credit from the Federal Home Loan Bank 0nrim_CertificatesofDepositsCollateralizedbyFederalHomeLoanBankLettersofCredit 0nrim_CertificatesofDepositsCollateralizedbyFederalHomeLoanBankLettersofCredit
Securities pledged to collateralize certificates of deposit 0us-gaap_DepositLiabilitiesCollateralIssuedFinancialInstruments 0us-gaap_DepositLiabilitiesCollateralIssuedFinancialInstruments
Deposits held for related parties 5,400,000us-gaap_RelatedPartyDepositLiabilities 12,500,000us-gaap_RelatedPartyDepositLiabilities
CDARS    
Deposits [Line Items]    
Number of banks in CDARS 3,000nrim_NumberOfBanksInCertificateOfDepositAccountRegistrySystem
/ dei_LegalEntityAxis
= nrim_CertificateOfDepositAccountRegistrySystemMember
 
Percentage of coverage of certificates of deposit in CDARS 100.00%nrim_PercentageOfCoverageOfCertificatesOfDepositInCertificateOfDepositAccountRegistrySystem
/ dei_LegalEntityAxis
= nrim_CertificateOfDepositAccountRegistrySystemMember
 
Certificates of deposit $ 1,006,000us-gaap_InterestBearingDomesticDepositCertificatesOfDeposits
/ dei_LegalEntityAxis
= nrim_CertificateOfDepositAccountRegistrySystemMember
$ 0us-gaap_InterestBearingDomesticDepositCertificatesOfDeposits
/ dei_LegalEntityAxis
= nrim_CertificateOfDepositAccountRegistrySystemMember