Note 6 - Loans and Allowance for Loan Losses - Number of Loans Modified Troubled Debt Restructuring by Loan Portfolio (Details) $ in Thousands	9 Months Ended	12 Months Ended
Sep. 30, 2016 USD ($)	Dec. 31, 2015 USD ($)
Real Estate [Member] \| Construction, Land Development and Other Land Loans [Member]
Number of Loans \| Security Loan	2	3
Pre-Modification Oustanding Principal Balance	$ 1,960	$ 2,220
Post-Modification Principal Balance	$ 1,341	$ 1,698
Real Estate [Member] \| Secured by 1-4 Family Residential Properties [Member]
Number of Loans \| Security Loan	4	4
Pre-Modification Oustanding Principal Balance	$ 335	$ 200
Post-Modification Principal Balance	$ 257	$ 103
Real Estate [Member] \| Secured by Non-farm, Non-residential Properties [Member]
Number of Loans \| Security Loan	2	2
Pre-Modification Oustanding Principal Balance	$ 113	$ 113
Post-Modification Principal Balance	$ 44	$ 52
Commercial and Industrial Loans [Member]
Number of Loans \| Security Loan	2	2
Pre-Modification Oustanding Principal Balance	$ 116	$ 116
Post-Modification Principal Balance	$ 90	$ 94
Number of Loans \| Security Loan	10	11
Pre-Modification Oustanding Principal Balance	$ 2,524	$ 2,649
Post-Modification Principal Balance	$ 1,732	$ 1,947

Number of financing receivables that have been modified by troubled debt restructurings.

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=66008989&loc=SL6953676-111524

+ Details

Name:	us-gaap_FinancingReceivableModificationsNumberOfContracts2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

+ References

+ Details

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Details

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=fusb_RealEstate1Member
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fusb_SecuredBy1To4FamilyResidentialPropertiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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