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Mortgage Servicing Rights - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended
--- | ---
Mar. 31, 2018 | Mar. 31, 2017
Other Noninterest Income |  |
Servicing Assets at Fair Value [Line Items] |  |
Mortgage servicing fees | $ 1.2 | $ 1.2

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- Definition Fees and Commissions, Mortgage Servicing + References No definition available. + Details
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Namespace Prefix: | nrim_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_ServicingAssetsAtFairValueLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Details
Name: | us-gaap_IncomeStatementLocationAxis=nrim_OtherNoninterestIncomeMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |