Name:	ubfo_FinancingReceivableModificationAllowanceForLoanLosses
Namespace Prefix:	ubfo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

Additions during period to recorded investment related to financing receivables modified by troubled debt restructuring.

No definition available.

Name:	ubfo_FinancingReceivableModificationsAdditions
Namespace Prefix:	ubfo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Amount of recorded investment related to financing receivables modified by troubled debt restructuring for which there was a payment default.

No definition available.

Name:	ubfo_FinancingReceivableModificationsDefaults
Namespace Prefix:	ubfo_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

Number of financing receivables that have been modified by troubled debt restructurings.

No definition available.

Name:	ubfo_FinancingReceivableModificationsNumberofContractsDuringPeriod2
Namespace Prefix:	ubfo_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

A roll forward is a reconciliation of a concept from the beginning of a period to the end of a period.

No definition available.

Name:	ubfo_FinancingReceivableModificationsRecordedInvestmentRollForward
Namespace Prefix:	ubfo_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

Financing Receivable, Period of Successful Payment History Used for Restructured Loan Accrual Status

No definition available.

Name:	ubfo_FinancingReceivablePeriodofSuccessfulPaymentHistoryUsedforRestructuredLoanAccrualStatus
Namespace Prefix:	ubfo_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

Number of financing receivables that have been restructured.

No definition available.

Name:	ubfo_NumberOfRestructuredLoans
Namespace Prefix:	ubfo_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

Amount of investment related to financing receivables restructured by troubled debt restructurings.

No definition available.

Name:	ubfo_TotalRestructuredLoans
Namespace Prefix:	ubfo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

-- None. No documentation exists for this element. --

No definition available.

Name:	ubfo_TroubledDebtRestructuringThatDefaultedAbstract
Namespace Prefix:	ubfo_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

-- None. No documentation exists for this element. --

No definition available.

Name:	ubfo_TroubledDebtRestructuringsAbstract
Namespace Prefix:	ubfo_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

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.

No definition available.

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

Post-modification amount of investment in financing receivables modified by troubled debt restructurings.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 33

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953676-111524

Name:	us-gaap_FinancingReceivableModificationsPostModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Pre-modification amount of investment in financing receivables modified by troubled debt restructurings.

Name:	us-gaap_FinancingReceivableModificationsPreModificationRecordedInvestment2
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

The amount of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 40

-Section 50

-Paragraph 4

-URI http://asc.fasb.org/extlink&oid=25495931&loc=d3e18845-111554

Name:	us-gaap_FinancingReceivableModificationsRecordedInvestment
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

Number of financing receivables that have been modified by troubled debt restructurings within the previous 12 months and for which there was a payment default.

Reference 1: http://www.xbrl.org/2003/role/presentationRef

-Publisher FASB

-Name Accounting Standards Codification

-Topic 310

-SubTopic 10

-Section 50

-Paragraph 34

-URI http://asc.fasb.org/extlink&oid=49124121&loc=SL6953682-111524

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultNumberOfContracts1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

Amount of investment related to financing receivables modified by troubled debt restructurings within the previous 12 months, for which there was a payment default.

Name:	us-gaap_FinancingReceivableModificationsSubsequentDefaultRecordedInvestment1
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Reflects the amount of the write-down related to a troubled debt restructuring.

Name:	us-gaap_FinancingReceivablesImpairedTroubledDebtRestructuringWriteDown
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialLoanMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ubfo_GovernmentProgramLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_CommercialRealEstateMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ubfo_SingleFamilyResidentialLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_HomeEquityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ubfo_AgriculturalMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=us-gaap_ConsumerPortfolioSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ubfo_OverdraftProtectionLinesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ubfo_TotalInstallmentAndOtherMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ubfo_TotalCommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: