Note 10 - Deposits - Interest Expense on Deposits (Details) - USD ($) $ in Thousands	12 Months Ended
Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Interest bearing checking accounts	$ 846	$ 856	$ 743
Money market accounts	821	790	449
Savings and certificate accounts	5,393	4,966	4,217
Early withdrawal penalties	(68)	(97)	(70)
Total	$ 6,992	$ 6,515	$ 5,339

Amount of interest expense on domestic savings and time deposit liabilities.

No definition available.

Name:	bsf_InterestExpenseDomesticDepositLiabilitiesSavingsAndTimeDeposits
Namespace Prefix:	bsf_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Aggregate amount of all interest expense on domestic deposit liabilities.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 225
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-04.6)
-URI http://asc.fasb.org/extlink&oid=6879574&loc=d3e536633-122882

+ Details

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

- Definition

Amount of interest expense on domestic checking deposit liabilities.

+ References

+ Details

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

- Definition

Early withdrawal penalties on domestic deposits which discourage customer withdrawal prior to maturity.

+ References

+ Details

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

- Definition

Interest expense incurred on all money market deposits.

+ References

+ Details

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