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v3.26.1

Note 6 - Fair Value Measurements (Details Textual) $ in Thousands | Jun. 30, 2026 USD ($) | Dec. 31, 2025 USD ($)
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Collateral, Market, Income, Enterprise, Liquidation Methods [Member] | Measurement Input, Cost to Sell [Member] | Minimum [Member] |  |
Impaired Loans, Measurement Input | 6 |
Collateral, Market, Income, Enterprise, Liquidation Methods [Member] | Measurement Input, Cost to Sell [Member] | Maximum [Member] |  |
Impaired Loans, Measurement Input | 10 |
Fair Value, Recurring [Member] |  |
Liabilities, Fair Value Disclosure | $ 0 | $ 0

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- Definition Value of input used to measure impaired loans. + References No definition available. + Details
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Namespace Prefix: | fnrn_
Data Type: | xbrli:pureItemType
Balance Type: | na
Period Type: | instant

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- Definition Fair value of financial and nonfinancial obligations. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (a) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 Reference 2: http://www.xbrl.org/2003/role/disclosureRef -Topic 820 -SubTopic 10 -Name Accounting Standards Codification -Section 50 -Paragraph 2 -Subparagraph (b) -Publisher FASB -URI https://asc.fasb.org/1943274/2147482106/820-10-50-2 + Details
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Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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

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

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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

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- Details
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Balance Type: |
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