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Interest Bearing Deposits - Composition of Network Deposits (Details) - USD ($) $ in Thousands | Jun. 30, 2024 | Dec. 31, 2023
--- | --- | ---
Interest Bearing Deposits [Line Items] |  |
Total network deposits | $ 700,272 | $ 636,524
CDARS |  |
Interest Bearing Deposits [Line Items] |  |
Total network deposits | 17,397 | 16,325
ICS |  |
Interest Bearing Deposits [Line Items] |  |
Total network deposits | $ 682,875 | $ 620,199

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- Definition Interest Bearing Deposits + References No definition available. + Details
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Data Type: | xbrli:stringItemType
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- Definition Total network deposits + References No definition available. + Details
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Namespace Prefix: | fsbc_
Data Type: | xbrli:monetaryItemType
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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: |