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Note 13 - Fair Value Measurement - Assets Measured at Fair Value on a Nonrecurring Basis (Details) - Fair Value, Nonrecurring [Member] - USD ($) $ in Thousands | Sep. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Individually evaluated collateral dependent loans | $ 27,181 | $ 17,388
Fair Value, Inputs, Level 1 [Member] |  |
Individually evaluated collateral dependent loans | 0 | 0
Fair Value, Inputs, Level 2 [Member] |  |
Individually evaluated collateral dependent loans | 0 | 0
Fair Value, Inputs, Level 3 [Member] |  |
Individually evaluated collateral dependent loans | $ 27,181 | $ 17,388

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- Definition The fair value of collateral dependent loans. + References No definition available. + Details
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Namespace Prefix: | fnwb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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
Name: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel1Member
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: | us-gaap_FairValueByFairValueHierarchyLevelAxis=us-gaap_FairValueInputsLevel3Member
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |