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Derivative Financial Instruments - Additional Information (Details) - USD ($)	Aug. 21, 2019	Aug. 09, 2019	Jul. 22, 2019	Jul. 08, 2019	Sep. 30, 2019
Forward Interest Rate Swap Contract One [Member]
Derivative [Line Items]
Notional amount, total				$ 10,000,000
Derivative term of contract				7 years
Derivative, fixed interest rate				1.79%
Forward Interest Rate Swap Contract One [Member] | Minimum [Member]
Derivative [Line Items]
Expected derivative notional amount for next seven years					$ 10,000,000
Forward Interest Rate Swap Contract Two [Member]
Derivative [Line Items]
Notional amount, total			$ 10,000,000
Derivative term of contract			5 years
Derivative, fixed interest rate			1.698%
Forward Interest Rate Swap Contract Two [Member] | Minimum [Member]
Derivative [Line Items]
Expected derivative notional amount for next five years					$ 10,000,000
Forward Interest Rate Swap Contract Three [Member]
Derivative [Line Items]
Notional amount, total		$ 10,000,000
Derivative term of contract		5 years
Derivative, fixed interest rate		1.406%
Forward Interest Rate Swap Contract Four [Member]
Derivative [Line Items]
Notional amount, total	$ 10,000,000
Derivative term of contract	5 years
Derivative, fixed interest rate	1.292%

X

- Definition

Expected derivative notional amount for next five years.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

X

- Definition

Expected derivative notional amount for next seven years.

+ References

No definition available.

+ Details

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Namespace Prefix:	fusb_
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X

- Definition

Nominal or face amount used to calculate payments on the derivative asset.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1B
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5580258-113959

Reference 3: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 1A
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109980867&loc=SL5579245-113959

+ Details

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X

- Definition

Fixed interest rate related to the interest rate derivative.

+ References

No definition available.

+ Details

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Balance Type:	na
Period Type:	instant

X

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

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X

- Definition

Period the derivative contract is outstanding, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

+ References

No definition available.

+ Details

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X

- Details

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- Details

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- Details

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- Details

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- Details

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