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Deposit Activities - Narrative (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Deposits [Abstract]    
Deposits $ 1,191,013,000us-gaap_Deposits $ 1,179,747,000us-gaap_Deposits
Certificates of deposit 147,300,000us-gaap_InterestBearingDomesticDepositCertificatesOfDeposits 142,200,000us-gaap_InterestBearingDomesticDepositCertificatesOfDeposits
CD maturities over next twelve months $ 75,100,000us-gaap_TimeDepositMaturitiesNextRollingTwelveMonths $ 81,900,000us-gaap_TimeDepositMaturitiesNextRollingTwelveMonths
CD maturities over next twelve months, percent 51.00%nrim_TimeDepositMaturitiesNextRollingTwelveMonthsPercentofTotalTimeDeposits 58.00%nrim_TimeDepositMaturitiesNextRollingTwelveMonthsPercentofTotalTimeDeposits