Note 4 - Loans and Allowance for Loan Losses (Details Textual) $ in Thousands	6 Months Ended	12 Months Ended
Jun. 30, 2018 USD ($)	Dec. 31, 2017 USD ($)	Mar. 31, 2018 USD ($)
Number of Portfolio Segments	8
Number of Loans, Related Party	0	0
Loans and Leases Receivable, Related Parties, Proceeds	$ 200
Financing Receivable, Recorded Investment, Nonaccrual Status	1,713	$ 2,148
Loans and Leases Receivable, Impaired, Interest Lost on Nonaccrual Loans	10	19
Interest Income Recorded	2	3
Financing Receivable, Restructuring Recorded Investment With Nonaccrual Status	100	100
Financing Receivable, Modifications, Recorded Investment	$ 0	$ 0
Financing Receivable, Modifications, Subsequent Default, Number of Contracts	0	0
Loans Modified in a Troubled Debt Restructuring, Individually Evaluated for Impairment, Principal Balance, Threshold			$ 500
Allowance for Credit Losses, Change in Method of Calculating Impairment	$ 2	$ 2
Minimum [Member]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	500
Minimum [Member] \| ALC [Member]
Loans and Leases Receivable, Nonperforming, Nonaccrual of Interest	100
Related Party Loan [Member]
Loans and Leases Receivable, Related Parties, Ending Balance	$ 400	500
Loans and Leases Receivable, Related Parties, Proceeds		$ 11
Real Estate [Member]
Percentage of Loan Portfolio	55.00%	54.00%

Represents the amount of financing receivable restructuring recorded investment with nonaccrual status.

No definition available.

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Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Interest income recorded.

+ References

No definition available.

+ Details

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Namespace Prefix:	fusb_
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Balance Type:	credit
Period Type:	instant

- Definition

Represents the threshold used in determining which loans that have been modified in a troubled debt restructuring are considered impaired and evaluated individually for impairment. All such loans with a principal balance above this threshold are considered impaired and evaluated individually for impairment.

+ References

No definition available.

+ Details

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- Definition

Indicates the number of loans made to related party during the period.

+ References

No definition available.

+ Details

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- Definition

Represents the number of portfolio segments into which the Company has divided the loan portfolio as of the balance sheet date.

+ References

No definition available.

+ Details

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Namespace Prefix:	fusb_
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- Definition

Represents the percentage of the loan portfolio that is concentrated in a certain category.

+ References

No definition available.

+ Details

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- Definition

Amount of allowance for credit losses related to receivables which had been measured under a general allowance for credit losses methodology that are now identified as troubled debt restructurings.

+ References

No definition available.

+ Details

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Balance Type:	credit
Period Type:	instant

- Definition

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18845-111554

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Balance Type:	debit
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- Definition

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=84173941&loc=SL6953682-111524

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- Definition

Recorded investment in financing receivables that are on nonaccrual status as of the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
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-SubTopic 10
-Section 50
-Paragraph 7
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-URI http://asc.fasb.org/extlink&oid=84173941&loc=d3e5111-111524

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Namespace Prefix:	us-gaap_
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Balance Type:	debit
Period Type:	instant

- 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://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=82895541&loc=d3e18794-111554

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Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Reflects the carrying amount of loans deemed to be questionable as to collection on which no interest is continuing to be recognized.

+ References

+ Details

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Balance Type:	debit
Period Type:	instant

- Definition

For an unclassified balance sheet, reflects the carrying amount of unpaid loan amounts due from related parties at the balance sheet date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 850
-SubTopic 10
-Section 50
-Paragraph 1
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Balance Type:	debit
Period Type:	instant

- Definition

Amount of cash inflow from related parties which reduce the amount of loans and leases receivable due from related parties.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 942
-SubTopic 210
-Section S99
-Paragraph 1
-Subparagraph (SX 210.9-03(7)(e)(1)(i))
-URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
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-Name Accounting Standards Codification
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-Section 45
-Paragraph 12
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-URI http://asc.fasb.org/extlink&oid=109223946&loc=d3e3179-108585

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Period Type:	duration

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Balance Type:
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- Details

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