Note 12 - Stock-based Compensation (Details) - Information about the Company’s Outstanding Options and Exercisable Options	12 Months Ended
Dec. 31, 2015 $ / shares shares
Note 12 - Stock-based Compensation (Details) - Information about the Company’s Outstanding Options and Exercisable Options [Line Items]
Outstanding Options, Shares (in Shares) \| shares	165,750
Outstanding Options, Life	5 years 54 days
Outstanding Options, Price	$ 10.76
Exercisable, Shares (in Shares) \| shares	112,450
Exercisable, Price	$ 12.70
Range A [Member]
Note 12 - Stock-based Compensation (Details) - Information about the Company’s Outstanding Options and Exercisable Options [Line Items]
Range of Exercise Prices, Lower	4.00
Range of Exercise Prices, Higher	$ 10.00
Outstanding Options, Shares (in Shares) \| shares	100,000
Outstanding Options, Life	6 years 295 days
Outstanding Options, Price	$ 6.16
Exercisable, Shares (in Shares) \| shares	46,700
Exercisable, Price	$ 5.56
Range B [Member].
Note 12 - Stock-based Compensation (Details) - Information about the Company’s Outstanding Options and Exercisable Options [Line Items]
Range of Exercise Prices, Lower	10.01
Range of Exercise Prices, Higher	$ 17.00
Outstanding Options, Shares (in Shares) \| shares	27,000
Outstanding Options, Life	3 years 255 days
Outstanding Options, Price	$ 11.01
Exercisable, Shares (in Shares) \| shares	27,000
Exercisable, Price	$ 11.01
Range C [Member]
Note 12 - Stock-based Compensation (Details) - Information about the Company’s Outstanding Options and Exercisable Options [Line Items]
Range of Exercise Prices, Lower	17.01
Range of Exercise Prices, Higher	$ 26.00
Outstanding Options, Shares (in Shares) \| shares	29,250
Outstanding Options, Life	2 years 94 days
Outstanding Options, Price	$ 20.65
Exercisable, Shares (in Shares) \| shares	29,750
Exercisable, Price	$ 20.65
Range D [Member]
Note 12 - Stock-based Compensation (Details) - Information about the Company’s Outstanding Options and Exercisable Options [Line Items]
Range of Exercise Prices, Lower	26.01
Range of Exercise Prices, Higher	$ 34.00
Outstanding Options, Shares (in Shares) \| shares	9,500
Outstanding Options, Life	251 days
Outstanding Options, Price	$ 28.14
Exercisable, Shares (in Shares) \| shares	9,500
Exercisable, Price	$ 28.14

No definition available.

Name:	fsbk_Note12StockbasedCompensationDetailsInformationabouttheCompanysOutstandingOptionsandExercisableOptionsLineItems
Namespace Prefix:	fsbk_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

The floor of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

The number of shares reserved for issuance pertaining to the outstanding exercisable stock options as of the balance sheet date in the customized range of exercise prices for which the market and performance vesting condition has been satisfied.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(iii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The number of shares reserved for issuance pertaining to the outstanding stock options as of the balance sheet date for all option plans in the customized range of exercise prices.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions
Namespace Prefix:	us-gaap_
Data Type:	xbrli:sharesItemType
Balance Type:	na
Period Type:	instant

- Definition

The ceiling of a customized range of exercise prices for purposes of disclosing shares potentially issuable under outstanding stock option awards on all stock option plans and other required information pertaining to awards in the customized range.

+ References

+ Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	duration

- Definition

Weighted average exercise price as of the balance sheet date for those equity-based payment arrangements exercisable and outstanding.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(iii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

The weighted average price as of the balance sheet date at which grantees could acquire the underlying shares with respect to all outstanding stock options which are in the customized range of exercise prices.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (c)(1)(i)-(ii)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1
Namespace Prefix:	us-gaap_
Data Type:	num:perShareItemType
Balance Type:	na
Period Type:	instant

- Definition

Weighted average remaining contractual term of outstanding stock options, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 718
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (e)(1)
-URI http://asc.fasb.org/extlink&oid=6415400&loc=d3e5070-113901

+ Details

Name:	us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=fsbk_RangeAMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=fsbk_RangeBMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=fsbk_RangeCMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis=fsbk_RangeDMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: