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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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Balance Type:	na
Period Type:	duration

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- Definition

Total network deposits

+ References

No definition available.

+ Details

Name:	fsbc_NetworkDeposits
Namespace Prefix:	fsbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

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- Details

Name:	srt_ProductOrServiceAxis=fsbc_CDARSMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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- Details

Name:	srt_ProductOrServiceAxis=fsbc_ICSMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: