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Investment Securities - Summary Of Investment Security Carrying and Fair Value (Details) (USD $)
In Thousands, unless otherwise specified
Mar. 31, 2015
Dec. 31, 2014
Schedule of Held-to-maturity Securities [Line Items]    
Investment securities held to maturity $ 2,199us-gaap_HeldToMaturitySecurities $ 2,201us-gaap_HeldToMaturitySecurities
Gross Unrealized Gains 97nrim_HeldtoMaturitySecuritiesAccumulatedUnrecognizedHoldingGain 107nrim_HeldtoMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
Gross Unrealized Losses 0nrim_HeldtoMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss 0nrim_HeldtoMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
Fair Value 2,296us-gaap_HeldToMaturitySecuritiesFairValue 2,308us-gaap_HeldToMaturitySecuritiesFairValue
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 253,579us-gaap_AvailableForSaleSecuritiesAmortizedCost 281,310us-gaap_AvailableForSaleSecuritiesAmortizedCost
Gross Unrealized Gains 1,243us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax 964us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
Gross Unrealized Losses 50us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax 544us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
Fair Value 254,772us-gaap_AvailableForSaleSecurities 281,730us-gaap_AvailableForSaleSecurities
U.S. Treasury and government sponsored entities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 199,754us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
226,624us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Gross Unrealized Gains 548us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
105us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Gross Unrealized Losses 47us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
539us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Fair Value 200,255us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
226,190us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USTreasuryAndGovernmentMember
Municipal securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 11,962us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
11,843us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
Gross Unrealized Gains 252us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
285us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
Gross Unrealized Losses 1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
4us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
Fair Value 12,213us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
12,124us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_MunicipalBondsMember
U.S. Agency mortgage-backed securities    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 976us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
1,024us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Gross Unrealized Gains 5us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
6us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Gross Unrealized Losses 2us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
1us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Fair Value 979us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
1,029us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_USGovernmentAgenciesDebtSecuritiesMember
Corporate bonds    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 38,895us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
38,820us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Gross Unrealized Gains 412us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
415us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Gross Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Fair Value 39,307us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
39,235us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_CorporateDebtSecuritiesMember
Preferred stock    
Schedule of Available-for-sale Securities [Line Items]    
Amortized Cost 1,992us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
2,999us-gaap_AvailableForSaleSecuritiesAmortizedCost
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
Gross Unrealized Gains 26us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
153us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedGainBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
Gross Unrealized Losses 0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
0us-gaap_AvailableForSaleSecuritiesAccumulatedGrossUnrealizedLossBeforeTax
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
Fair Value 2,018us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
3,152us-gaap_AvailableForSaleSecurities
/ us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis
= us-gaap_PreferredStockMember
Municipal securities    
Schedule of Held-to-maturity Securities [Line Items]    
Investment securities held to maturity 2,199us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= us-gaap_MunicipalBondsMember
2,201us-gaap_HeldToMaturitySecurities
/ us-gaap_DebtSecurityAxis
= us-gaap_MunicipalBondsMember
Gross Unrealized Gains 97nrim_HeldtoMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_DebtSecurityAxis
= us-gaap_MunicipalBondsMember
107nrim_HeldtoMaturitySecuritiesAccumulatedUnrecognizedHoldingGain
/ us-gaap_DebtSecurityAxis
= us-gaap_MunicipalBondsMember
Gross Unrealized Losses 0nrim_HeldtoMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_MunicipalBondsMember
0nrim_HeldtoMaturitySecuritiesAccumulatedUnrecognizedHoldingLoss
/ us-gaap_DebtSecurityAxis
= us-gaap_MunicipalBondsMember
Fair Value $ 2,296us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_MunicipalBondsMember
$ 2,308us-gaap_HeldToMaturitySecuritiesFairValue
/ us-gaap_DebtSecurityAxis
= us-gaap_MunicipalBondsMember