Amount of allowance to cover probable credit losses on loans and leases. Includes carryover of or adjustments to the allowance for loan losses in connection with business combinations. Excludes allowance for loans and leases covered under loss sharing agreements.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7(d))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (g)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 30
-Section 50
-Paragraph 2
-Subparagraph (b)(1)(ii)
-URI http://asc.fasb.org/extlink&oid=82894903&loc=d3e15032-111544

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11B
-Subparagraph (c)(1)
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953423-111524

Reference 5: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section S99
-Paragraph 4
-Subparagraph (SAB Topic 6.L.1)
-URI http://asc.fasb.org/extlink&oid=27010918&loc=d3e74567-122707

+ Details

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

- Definition

Amount of deferred interest and fee income, unamortized costs incurred to originate loans and leases, unamortized loan commitments and loan syndication fees, and premiums over or discounts from face amounts of acquired loans. Excludes amounts for loans and leases covered under loss sharing agreements.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 840
-SubTopic 30
-Section 50
-Paragraph 4
-Subparagraph (a)(4)
-URI http://asc.fasb.org/extlink&oid=84165051&loc=d3e45377-112738

Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5074-111524

Reference 4: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 20
-Section 50
-Paragraph 3
-URI http://asc.fasb.org/extlink&oid=84159169&loc=d3e10152-111534

+ Details

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

- Definition

Amount before allowance of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Includes deferred interest and fees, undisbursed portion of loan balance, unamortized costs and premiums and discounts from face amounts. Excludes loans and leases covered under loss sharing agreements.

+ References

+ Details

Name:	us-gaap_LoansAndLeasesReceivableGrossCarryingAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Amount after allowance and deduction of deferred interest and fees, unamortized costs and premiums and discounts from face amounts, of loans and leases held in portfolio, including but not limited to, commercial and consumer loans. Excludes loans and leases covered under loss sharing agreements and loans held for sale.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=82891408&loc=d3e4428-111522

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03.7)
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

+ Details

Name:	us-gaap_LoansAndLeasesReceivableNetReportedAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=fusb_FirstUnitedSecurityBankMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=fusb_AcceptanceLoanCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- 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_SecuredByMultifamilyResidentialPropertiesMember
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_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=fusb_OtherRealEstateLoansMember
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:

- Details

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

- Details

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

- Details

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

Note 4 - Loans and Allowance for Loan Losses - Loan Portfolio (Details) - USD ($) $ in Thousands	Jun. 30, 2018	Dec. 31, 2017	Jun. 30, 2017	Dec. 31, 2016
Total loans	$ 367,115	$ 357,458
Less: Unearned interest, fees and deferred cost	6,634	6,563
Loans, allowance for loan losses	4,952	4,774		$ 4,856
Net loans	355,529	346,121	$ 330,526
FUSB [Member]
Total loans	257,608	257,503
Less: Unearned interest, fees and deferred cost	351	374
Loans, allowance for loan losses	2,520	2,447		2,409
Net loans	254,737	254,682
ALC [Member]
Total loans	109,507	99,955
Less: Unearned interest, fees and deferred cost	6,283	6,189
Loans, allowance for loan losses	2,432	2,327		2,447
Net loans	100,792	91,439
Real Estate [Member] \| Construction, Land Development and Other Land Loans [Member]
Total loans	22,878	26,143
Loans, allowance for loan losses	166	203		535
Real Estate [Member] \| Construction, Land Development and Other Land Loans [Member] \| FUSB [Member]
Total loans	22,878	26,143
Loans, allowance for loan losses	166	203		535
Real Estate [Member] \| Construction, Land Development and Other Land Loans [Member] \| ALC [Member]
Total loans
Loans, allowance for loan losses
Real Estate [Member] \| Secured by 1-4 Family Residential Properties [Member]
Total loans	48,144	45,073
Loans, allowance for loan losses	295	290		411
Real Estate [Member] \| Secured by 1-4 Family Residential Properties [Member] \| FUSB [Member]
Total loans	38,968	34,272
Loans, allowance for loan losses	271	238		304
Real Estate [Member] \| Secured by 1-4 Family Residential Properties [Member] \| ALC [Member]
Total loans	9,176	10,801
Loans, allowance for loan losses	24	52		107
Real Estate [Member] \| Secured by Multi-family Residential Properties [Member]
Total loans	18,374	16,579
Loans, allowance for loan losses	128	116		88
Real Estate [Member] \| Secured by Multi-family Residential Properties [Member] \| FUSB [Member]
Total loans	18,374	16,579
Loans, allowance for loan losses	128	116		88
Real Estate [Member] \| Secured by Multi-family Residential Properties [Member] \| ALC [Member]
Total loans
Loans, allowance for loan losses
Real Estate [Member] \| Secured by Non-farm, Non-residential Properties [Member]
Total loans	112,461	105,133
Loans, allowance for loan losses	808	777		903
Real Estate [Member] \| Secured by Non-farm, Non-residential Properties [Member] \| FUSB [Member]
Total loans	112,461	105,133
Loans, allowance for loan losses	808	777		903
Real Estate [Member] \| Secured by Non-farm, Non-residential Properties [Member] \| ALC [Member]
Total loans
Loans, allowance for loan losses
Real Estate [Member] \| Other Real Estate Loans [Member]
Total loans	187	190
Loans, allowance for loan losses	2	2		2
Real Estate [Member] \| Other Real Estate Loans [Member] \| FUSB [Member]
Total loans	187	190
Loans, allowance for loan losses	2	2		2
Real Estate [Member] \| Other Real Estate Loans [Member] \| ALC [Member]
Total loans
Loans, allowance for loan losses
Commercial and Industrial Loans [Member]
Total loans	59,320	69,969
Loans, allowance for loan losses	1,098	1,049		527
Commercial and Industrial Loans [Member] \| FUSB [Member]
Total loans	59,320	69,969
Loans, allowance for loan losses	1,098	1,049		527
Commercial and Industrial Loans [Member] \| ALC [Member]
Total loans
Loans, allowance for loan losses
Consumer Portfolio Segment [Member] \| FUSB [Member]
Total loans	5,420	5,217
Consumer Portfolio Segment [Member] \| Consumer Loans [Member]
Total loans	38,322	39,300
Loans, allowance for loan losses	1,770	1,715		1,767
Consumer Portfolio Segment [Member] \| Consumer Loans [Member] \| FUSB [Member]
Total loans	5,420	5,217
Loans, allowance for loan losses	47	62		50
Consumer Portfolio Segment [Member] \| Consumer Loans [Member] \| ALC [Member]
Total loans	32,902	34,083
Loans, allowance for loan losses	1,723	1,653		1,717
Consumer Portfolio Segment [Member] \| Indirect Sales [Member]
Total loans	67,429	55,071
Loans, allowance for loan losses	685	622		623
Consumer Portfolio Segment [Member] \| Indirect Sales [Member] \| FUSB [Member]
Total loans
Loans, allowance for loan losses
Consumer Portfolio Segment [Member] \| Indirect Sales [Member] \| ALC [Member]
Total loans	67,429	55,071
Loans, allowance for loan losses	$ 685	$ 622		$ 623