Name:	bsf_FundingPeriod
Namespace Prefix:	bsf_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Name:	bsf_Note17OffBalanceSheetArrangementsandCommitmentsDetailsLineItems
Namespace Prefix:	bsf_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

The total amount of overdraft protection limits that have been opted in by customers and available for use on demand deposit and savings accounts.

No definition available.

Name:	bsf_OverdraftProtection
Namespace Prefix:	bsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

The percentage of predetermined overdraft limits that have been used by deposit account holders.

No definition available.

Name:	bsf_PercentOfLimit
Namespace Prefix:	bsf_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	instant

The total amount of the contingent obligation under letters of credit outstanding as of the reporting date.

No definition available.

Name:	us-gaap_LettersOfCreditOutstandingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Amount of borrowing capacity currently available under the credit facility (current borrowing capacity less the amount of borrowings outstanding).

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 210

-SubTopic 10

-Section S99

-Paragraph 1

-Subparagraph (SX 210.5-02.19(b),22(b))

-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher SEC

-Name Regulation S-X (SX)

-Number 210

-Section 02

-Paragraph 19, 22

-Article 5

Name:	us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Reflects the amount of cash required to fully fund agreements to provide advances to borrowers and lessees at stated interest rates.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 825

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6480020&loc=d3e61082-112788

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 825

-Section 50

-Paragraph 1

-Subparagraph (b)(2)

-URI http://asc.fasb.org/extlink&oid=6480020&loc=d3e61044-112788

Name:	us-gaap_LoansAndLeasesReceivableCommitmentsFixedRates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Reflects the carrying amount of loans and leases for which agreements to acquire or dispose exist.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 825

-Section 50

-Paragraph 2

-URI http://asc.fasb.org/extlink&oid=6480020&loc=d3e61082-112788

Reference 2: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 942

-SubTopic 825

-Section 50

-Paragraph 1

-Subparagraph (a)

-URI http://asc.fasb.org/extlink&oid=6480020&loc=d3e61044-112788

Name:	us-gaap_LoansAndLeasesReceivableCommitmentsToPurchaseOrSell
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Amount of undisbursed portion of the loan receivable balance (loans in process). Excludes loans and leases covered under loss sharing agreements.

No definition available.

Name:	us-gaap_LoansAndLeasesReceivableLoansInProcess
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Name:	us-gaap_CreditFacilityAxis=us-gaap_LetterOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_CreditFacilityAxis=us-gaap_StandbyLettersOfCreditMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_DerivativeMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancialInstrumentAxis=us-gaap_InterestRateLockCommitmentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bsf_CommitmentToSellMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bsf_ConstructionLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=bsf_MortgageLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_RangeAxis=us-gaap_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: