Name:	fsbc_InterestExpenseDomesticDepositLiabilitiesTimeDeposit250000OrMore
Namespace Prefix:	fsbc_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Aggregate amount of interest expense on all deposits.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 220
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.6)
-URI http://asc.fasb.org/extlink&oid=120399700&loc=SL114874048-224260

Name:	us-gaap_InterestExpenseDeposits
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of interest expense on domestic checking deposit liabilities.

Name:	us-gaap_InterestExpenseDomesticDepositLiabilitiesChecking
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of interest expense on domestic money market demand account deposit liabilities.

Name:	us-gaap_InterestExpenseDomesticDepositLiabilitiesMoneyMarket
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of interest expense on domestic time deposit liabilities other than certificates of liabilities.

Name:	us-gaap_InterestExpenseDomesticDepositLiabilitiesOtherTimeDeposit
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of interest expense on domestic savings deposit liabilities.