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Other Real Estate Owned and Repossessed Assets - Summary of Foreclosed Property Activity (Details) - USD ($) $ in Thousands	3 Months Ended
	Mar. 31, 2023	Mar. 31, 2022
Other Real Estate [Abstract]
Beginning balance	$ 686	$ 2,149
Sales proceeds		(1,431)
Gross gains		183
Gross losses		(27)
Net gains (losses)		156
Impairment	(69)
Ending balance	$ 617	$ 874

X

- Definition

Other real estate gross gains.

+ References

No definition available.

+ Details

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Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

Other real estate gross losses.

+ References

No definition available.

+ Details

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Balance Type:	debit
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X

- Definition

Other real estate net income loss.

+ References

No definition available.

+ Details

Name:	fusb_OtherRealEstateNetIncomeLoss
Namespace Prefix:	fusb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

X

- Definition

The charge against earnings in the period to reduce the carrying amount of real property to fair value.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 28
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3602-108585

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 360
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (b)
-URI https://asc.fasb.org/extlink&oid=109226691&loc=d3e2921-110230

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X

- References

No definition available.

+ Details

Name:	us-gaap_OtherRealEstateAbstract
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Balance Type:	na
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X

- Definition

The cash inflow associated with the sale of other real estate not otherwise defined in the taxonomy.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 12
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3179-108585

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X

- Definition

Carrying amount as of the balance sheet date of land and buildings obtained through foreclosure proceedings or defeasance in full or partial satisfaction of a debt arrangement.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 11
-URI https://asc.fasb.org/extlink&oid=123577603&loc=d3e5162-111524

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 45
-Paragraph 3
-URI https://asc.fasb.org/extlink&oid=124259787&loc=d3e4437-111522

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