Derivative instrument, interest rate, increase (decrease).

No definition available.

Name:	fusb_DerivativeInstrumentInterestRateIncreaseDecrease
Namespace Prefix:	fusb_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Number of credit risk participation agreement.

+ References

No definition available.

+ Details

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

- Definition

Number of interest rate swap contracts.

+ References

No definition available.

+ Details

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

- 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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-5

+ Details

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

- Definition

Date derivative contract ends, in YYYY-MM-DD format.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (e)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478795/946-210-50-6

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section 55
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477439/946-210-55-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 210
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478795/946-210-50-6

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 5
-Subparagraph (SX 210.12-13(Column A)(Footnote 3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477271/946-320-S99-5

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 5C
-Subparagraph (SX 210.12-13C(Column D))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477271/946-320-S99-5C

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 5
-Subparagraph (SX 210.12-13(Column F))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477271/946-320-S99-5

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 5A
-Subparagraph (SX 210.12-13A(Column C))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477271/946-320-S99-5A

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 5B
-Subparagraph (SX 210.12-13B(Column D))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477271/946-320-S99-5B

+ Details

Name:	us-gaap_DerivativeMaturityDates
Namespace Prefix:	us-gaap_
Data Type:	xbrli:dateItemType
Balance Type:	na
Period Type:	duration

- Definition

Nominal or face amount used to calculate payment on derivative.

+ References

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1B

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1A

+ Details

Name:	us-gaap_DerivativeNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	na
Period Type:	instant

- Definition

The net change in the difference between the fair value and the carrying value, or in the comparative fair values, of derivative instruments, including options, swaps, futures, and forward contracts, held at each balance sheet date, that was included in earnings for the period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 45
-Paragraph 28
-Subparagraph (b)
-SubTopic 10
-Topic 230
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-28

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(7)(c)(3))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(7)(c)(5))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(7)(c)(6))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 220
-Name Accounting Standards Codification
-Section S99
-Paragraph 1
-Subparagraph (SX 210.6-07(7)(c)(7))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479134/946-220-S99-1

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 946
-SubTopic 320
-Name Accounting Standards Codification
-Section S99
-Paragraph 5A
-Subparagraph (SX 210.12-13A(Column F))
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147477271/946-320-S99-5A

+ Details

Name:	us-gaap_UnrealizedGainLossOnDerivatives
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateSwapMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_CashFlowHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=us-gaap_InterestRateFloorMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fusb_ForwardInterestRateSwapContractsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fusb_ForwardInterestRateSwapContractsOneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fusb_ForwardInterestRateSwapContractsTwoMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_NetInvestmentHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentRiskAxis=fusb_CreditDerivativesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_HedgingDesignationAxis=us-gaap_DesignatedAsHedgingInstrumentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_DerivativeInstrumentsGainLossByHedgingRelationshipAxis=us-gaap_FairValueHedgingMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Derivative Financial Instruments - Additional Information (Details) $ in Millions				1 Months Ended			12 Months Ended
	Jun. 30, 2025 USD ($)	Dec. 31, 2024 USD ($)	Jan. 31, 2023 USD ($)	Mar. 31, 2025 USD ($) Contract	Aug. 31, 2024 USD ($) Contract	Mar. 31, 2024 USD ($)	Dec. 31, 2024 USD ($) CreditRisk
Derivative [Line Items]
Number of credit risk participation agreement \| CreditRisk							3
Interest Rate Swap [Member]
Derivative [Line Items]
Derivative, maturity dates	Feb. 28, 2023	Feb. 28, 2023
Net unrealized gain	$ 0.2	$ 0.3
Cash Flow Hedging [Member] \| Interest Rate Floor [Member]
Derivative [Line Items]
Notional amount				$ 20.0		$ 25.0
Derivative, maturity dates				Mar. 15, 2029		Mar. 27, 2026
Cash Flow Hedging [Member] \| Interest Rate Floor [Member] \| Maximum [Member]
Derivative [Line Items]
Derivative instrument, interest rate, increase (decrease)				(3.05%)		(4.00%)
Cash Flow Hedging [Member] \| Forward Interest Rate Swap Contracts [Member]
Derivative [Line Items]
Number of interest rate swap contracts \| Contract					2
Cash Flow Hedging [Member] \| Forward Interest Rate Swap Contracts One [Member]
Derivative [Line Items]
Notional amount					$ 20.0
Derivative, maturity dates					Sep. 06, 2027
Cash Flow Hedging [Member] \| Forward Interest Rate Swap Contracts Two [Member]
Derivative [Line Items]
Notional amount					$ 40.0
Derivative, maturity dates					Mar. 15, 2028
Cash Flow Hedging [Member] \| Interest Rate Swap [Member]
Derivative [Line Items]
Notional amount			$ 10.0
Derivative, maturity dates			Jul. 08, 2026
Net Investment Hedging [Member] \| Credit Derivatives [Member]
Derivative [Line Items]
Notional amount		$ 15.2					$ 15.2
Derivatives Designated as Hedging Instruments [Member]
Derivative [Line Items]
Number of interest rate swap contracts \| Contract				3
Notional amount				$ 30.0
Derivatives Designated as Hedging Instruments [Member] \| Fair Value Hedging [Member]
Derivative [Line Items]
Notional amount				$ 10.0