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Interest-Bearing Deposits - Narrative (Details) - USD ($) | Jun. 30, 2024 | Dec. 31, 2023
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Interest-bearing Deposits |  |
Interest-bearing domestic deposit, time deposits | $ 333,431,000 | $ 466,600,000
One-way deposits | $ 0 | $ 0

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- Definition Interest-bearing Deposits + References No definition available. + Details
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Namespace Prefix: | fsbc_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition One-Way Deposits + References No definition available. + Details
Name: | fsbc_OneWayDeposits
Namespace Prefix: | fsbc_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Amount of interest-bearing domestic time deposits. Time deposits bear interest for a fixed, stated period of time and include certificate of deposits and individual retirement accounts. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Topic 942 -SubTopic 210 -Name Accounting Standards Codification -Section S99 -Paragraph 1 -Subparagraph (SX 210.9-03(12)) -Publisher FASB -URI https://asc.fasb.org/1943274/2147478546/942-210-S99-1 + Details
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Balance Type: | credit
Period Type: | instant