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Purchased Receivables - Narrative (Details) | 12 Months Ended
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Dec. 31, 2021 contract purchased_receivable | Dec. 31, 2020 purchased_receivable contract | Dec. 31, 2019 contract
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Maturity of purchased receivables (less than) | 1 year |  |
Number of purchased receivables past due | purchased_receivable | 0 | 0 |
Purchased Receivable |  |  |
Accounts, Notes, Loans and Financing Receivable [Line Items] |  |  |
Number of restructured purchased receivables | contract | 0 | 0 | 0

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- Definition Purchased Receivable, Past Due, Number + References No definition available. + Details
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Namespace Prefix: | nrim_
Data Type: | xbrli:integerItemType
Balance Type: | na
Period Type: | instant

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- Definition Purchased Receivables, Term Of Receivable + References No definition available. + Details
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Namespace Prefix: | nrim_
Data Type: | xbrli:durationItemType
Balance Type: | na
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
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Number of financing receivables that have been modified by troubled debt restructurings. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 310 -SubTopic 10 -Section 50 -Paragraph 33 -URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524 + Details
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Namespace Prefix: | us-gaap_
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Balance Type: | na
Period Type: | duration

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |