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Restructuring Charges - Additional Information (Details)	3 Months Ended
	Sep. 30, 2021 Branch
ALC [Member]
Restructuring Cost And Reserve [Line Items]
Number of office closed	20

X

- Definition

Represents number of office closed during period for restructuring and related cost.

+ References

No definition available.

+ Details

Name:	fusb_RestructuringAndRelatedCostNumberOfOfficeClosed
Namespace Prefix:	fusb_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

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

Name:	us-gaap_RestructuringCostAndReserveLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

X

- Details

Name:	us-gaap_RestructuringCostAndReserveAxis=fusb_AcceptanceLoanCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: