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Note 14 - Guarantees, Commitments and Contingencies (Details Textual) - USD ($) $ in Thousands | 9 Months Ended |
--- | --- | ---
Sep. 30, 2017 | Sep. 30, 2016 | Dec. 31, 2016
Credit Losses Associated With Derivative Contracts | $ 0 | $ 0 |
Securities Committed to Purchase Amount | 0 |  | $ 0
Securities Committed to Sell Amount | 0 |  | $ 0
Construction Management Agreement [Member] |  |  |
Guarantor Obligations, Maximum Exposure, Undiscounted | $ 300 |  |

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- Definition Credit losses associated with derivative contracts. + References No definition available. + Details
Name: | fusb_CreditLossesAssociatedWithDerivativeContracts
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Definition Securities committed to purchase amount. + References No definition available. + Details
Name: | fusb_SecuritiesCommittedToPurchaseAmount
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Securities committed to sell amount. + References No definition available. + Details
Name: | fusb_SecuritiesCommittedToSellAmount
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Definition Maximum potential amount of future payments (undiscounted) the guarantor could be required to make under the guarantee or each group of similar guarantees before reduction for potential recoveries under recourse or collateralization provisions. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 460 -SubTopic 10 -Section 50 -Paragraph 4 -Subparagraph (b)(1) -URI http://asc.fasb.org/extlink&oid=82913245&loc=d3e12069-110248 + Details
Name: | us-gaap_GuaranteeObligationsMaximumExposure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | instant

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- Details
Name: | us-gaap_GuaranteeObligationsByNatureAxis=fusb_ConstructionManagementAgreementMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |