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Cash and Cash Equivalents (Details) | 9 Months Ended
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Sep. 30, 2017 USD ($)
Cash and Due from Banks [Abstract] |
Required daily balance with Federal Reserve | $ 3,600,000
Required balance | 500,000
Compensating balance, initial margin on interest rate swap | 100,000
Compensating balance, fair value exposure on interest rate swap | $ 200,000

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- Definition Compensating Balance, Amount, Fair Value Exposure On Interest Rate Swap + References No definition available. + Details
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Namespace Prefix: | nrim_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Compensating Balance, Amount, Initial Margin On Interest Rate Swap + References No definition available. + Details
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Namespace Prefix: | nrim_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Federal Reserve Bank Required Minimum Average daily Balance + References No definition available. + Details
Name: | nrim_FederalReserveBankRequiredMinimumAverageDailyBalance
Namespace Prefix: | nrim_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- References No definition available. + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
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- Definition The amount of a specific compensating balance arrangement that is maintained under an agreement for a bank loan or future credit availability. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 210 -SubTopic 10 -Section S99 -Paragraph 1 -Subparagraph (SX 210.5-02.1) -URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682 + Details
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Period Type: | instant