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

Financing Receivables, Impaired, Troubled Debt Restructuring, Credit Losses Write Offs

No definition available.

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

Financing Receivables, Impaired, Troubled Debt Restructuring, Principal Additions (Reductions)

No definition available.

Name:	ubfo_FinancingReceivablesImpairedTroubledDebtRestructuringPrincipalAdditionsReductions
Namespace Prefix:	ubfo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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_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

Amount, after troubled debt restructuring, of modified financing receivable.

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

Amount, before troubled debt restructuring, of financing receivable to be modified.

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

Amount of financing receivable modified as troubled debt restructuring.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 33
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953676-111524

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 40
-Section 50
-Paragraph 4
-URI http://asc.fasb.org/extlink&oid=121585226&loc=d3e18845-111554

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

Number of financing receivable contracts modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

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

Amount of financing receivable modified as troubled debt restructuring within previous 12 months, with subsequent payment default.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 310
-SubTopic 10
-Section 50
-Paragraph 34
-Subparagraph (a)(2)
-URI http://asc.fasb.org/extlink&oid=123577603&loc=SL6953682-111524

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

Name:	us-gaap_FinancingReceivablePortfolioSegmentAxis=ubfo_CommercialAndIndustrialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Name:	us-gaap_FinancingReceivableRecordedInvestmentByClassOfFinancingReceivableAxis=ubfo_CommercialAndBusinessLoansMember
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_FinancingReceivablePortfolioSegmentAxis=ubfo_RealEstateMortgageMember
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=us-gaap_ResidentialMortgageMember
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_FinancingReceivablePortfolioSegmentAxis=us-gaap_ConstructionLoansMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

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

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

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