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Note 15 - Fair Value of Financial Instruments - Assets and Liabilities Measured at Fair Value Using Significant Unobservable Inputs (Level 3) (Details) - Fair Value, Inputs, Level 3 [Member] $ in Thousands | Mar. 31, 2018 USD ($)
--- | ---
Fair value | $ 691
Fair value | 3,343
Assets held for sale | $ 228
Minimum [Member] | Measurement Input, Comparability Adjustment [Member] |
Quantitative range of unobservable inputs | 9
Quantitative range of unobservable inputs | 9
Assets held for sale | 9
Maximum [Member] | Measurement Input, Comparability Adjustment [Member] |
Quantitative range of unobservable inputs | 10
Quantitative range of unobservable inputs | 10
Assets held for sale | 10
Weighted Average [Member] | Measurement Input, Comparability Adjustment [Member] |
Quantitative range of unobservable inputs | 9.5
Quantitative range of unobservable inputs | 9.5
Assets held for sale | (9.5)

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- Definition Fair value portion of impaired loans by an entity as a result of past transactions or events. + References No definition available. + Details
Name: | fusb_ImpairedLoansFairValue
Namespace Prefix: | fusb_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Value of input used to measure impaired loans. + References No definition available. + Details
Name: | fusb_ImpairedLoansMeasurementInput
Namespace Prefix: | fusb_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Definition Value of input used to measure other assets. + References No definition available. + Details
Name: | fusb_OtherAssetsMeasurementInput
Namespace Prefix: | fusb_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Definition Fair value portion of other assets. + References No definition available. + Details
Name: | us-gaap_OtherAssetsFairValueDisclosure
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

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- Definition Carrying amount of other real estate not separately disclosed, including, but not limited to, real estate investments, real estate loans that qualify as investments in real estate, and premises that are no longer used in operations may also be included in real estate owned. This does not include real estate assets taken in settlement of troubled loans through surrender or foreclosure. + 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 210.9-03.10(4)) -URI http://asc.fasb.org/extlink&oid=6876686&loc=d3e534808-122878 + Details
Name: | us-gaap_OtherRealEstate
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | instant

X
- Definition Value of input used to measure real estate owned by financial institution by financial institution which is not directly related to its business. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 820 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (bbb)(2) -URI http://asc.fasb.org/extlink&oid=99377789&loc=d3e19207-110258 + Details
Name: | us-gaap_OtherRealEstateOwnedMeasurementInput
Namespace Prefix: | us-gaap_
Data Type: | xbrli:decimalItemType
Balance Type: | na
Period Type: | instant

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- Details
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Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MinimumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | us-gaap_MeasurementInputTypeAxis=us-gaap_MeasurementInputComparabilityAdjustmentMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_MaximumMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |

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- Details
Name: | srt_RangeAxis=srt_WeightedAverageMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |