Note 5 - Impaired Loans Interest Income (Details) - USD ($)	3 Months Ended	6 Months Ended
Jun. 30, 2016	Jun. 30, 2015	Jun. 30, 2016	Jun. 30, 2015
Real Estate Portfolio Segment [Member] \| One-to-Four-Family Residential [Member]
Average Recorded Investment-with a valuation allowance	$ 443,000	$ 901,000	$ 399	$ 1,002
Interest Income Recognized-with a valuation allowance	1,000	2,000	3	4
Average Recorded Investment-without a valuation allowance	7,870,000	4,345,000	7,362	4,309
Interest Income Recognized-without a valuation allowance	0	0	0	1
Real Estate Portfolio Segment [Member] \| Nonfarm Nonresidential [Member]
Average Recorded Investment-with a valuation allowance	715,000	2,179,000	1,275	2,194
Interest Income Recognized-with a valuation allowance	0	0	0	0
Average Recorded Investment-without a valuation allowance	4,924,000	929,000	4,697	915
Interest Income Recognized-without a valuation allowance	0	0	0	0
Real Estate Portfolio Segment [Member] \| Farmland [Member]
Average Recorded Investment-with a valuation allowance	0	480,000	158	481
Interest Income Recognized-with a valuation allowance	0	0	0	0
Average Recorded Investment-without a valuation allowance	817,000	250,000	711	250
Interest Income Recognized-without a valuation allowance	0	0	0	0
Real Estate Portfolio Segment [Member] \| Construction Loans [Member]
Average Recorded Investment-with a valuation allowance	83,000	106,000	126	126
Interest Income Recognized-with a valuation allowance	0	0	0	0
Average Recorded Investment-without a valuation allowance	681,000	527,000	617	531
Interest Income Recognized-without a valuation allowance	2,000	1,000	3	3
Real Estate Portfolio Segment [Member] \| Multifamily Residential [Member]
Average Recorded Investment-without a valuation allowance	153,000	0	179	0
Interest Income Recognized-without a valuation allowance	0	0	0	0
Commercial Portfolio Segment [Member]
Average Recorded Investment-with a valuation allowance	3,261,000	0	2,557	0
Interest Income Recognized-with a valuation allowance	0	0	0	0
Average Recorded Investment-without a valuation allowance	942,000	576,000	1,656	486
Interest Income Recognized-without a valuation allowance	0	0	0	0
Consumer Portfolio Segment [Member]
Average Recorded Investment-with a valuation allowance	5,000	15,000	5	15
Interest Income Recognized-with a valuation allowance	0	0	0	0
Average Recorded Investment-without a valuation allowance	245,000	104,000	224	75
Interest Income Recognized-without a valuation allowance	0	0	0	0
Average Recorded Investment-with a valuation allowance	4,507,000	3,681,000	4,520	3,818
Interest Income Recognized-with a valuation allowance	1,000	2,000	3	4
Average Recorded Investment-without a valuation allowance	15,632,000	6,731,000	15,446	6,566
Interest Income Recognized-without a valuation allowance	2,000	1,000	3	4
Total impaired loans	20,139,000	10,412,000	19,966	10,384
Total impaired loans	3,000	3,000	6	8
Interest based on original terms	299,000	189,000	604	389
Interest income recognized on a cash basis on impaired loans	$ 0	$ 0	$ 0	$ 0

Average amount of investment of impaired financing receivables with related allowance for credit losses and without a related allowance for credit losses.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5258-111524

+ Details

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

- Definition

Amount of interest income recognized that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

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

- Definition

Amount of interest income recognized on a cash-basis method of accounting that the financing receivables (with related allowance for credit losses and without a related allowance for credit losses) were impaired.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is no related allowance for credit losses.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 17
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5258-111524

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 15
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=66008989&loc=d3e5212-111524

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables that are recognized as impaired, for which there is no related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of average investment of impaired financing receivables for which there is a related allowance for credit losses.

+ References

+ Details

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

- Definition

Amount of interest income, recognized on accrual basis, on financing receivables, for which there is a related allowance for credit losses and are recognized as impaired.

+ References

+ Details

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

- Definition

Reflects the amount of additional interest income that would have been recorded if impaired or nonperforming loans were instead current, in compliance with their original terms, and outstanding throughout the reporting period or since origination (if held for part of the period).

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18794-111554

+ Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=bsf_RealEstatePortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_CommercialPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: