Loans - TDR Activity by Loan Category (Details) $ in Thousands	3 Months Ended	6 Months Ended
Jun. 30, 2020 USD ($) loan	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) contract payment_deferral loan	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($) loan
Financing Receivable, Troubled Debt Restructuring [Line Items]
Financing receivable, period of successful payment history used for restructured loan accrual status			6 months
Troubled Debt Restructurings
Number of Contracts \| contract			1
Pre- Modification Outstanding Recorded Investment			$ 179
Post- Modification Outstanding Recorded Investment			$ 179
Number of Contracts which Defaulted During Period \| contract			0
Recorded Investment on Defaulted TDRs			$ 0
Number of restructured loans \| loan	13		13		13
Total restructured loans	$ 5,155		$ 5,155		$ 5,187
TDR activity by loan category [Roll Forward]
Beginning balance	5,253	$ 6,340	5,187	$ 7,059
Additions	0	0	179	0
Principal (reductions) additions	(98)	(157)	(211)	(871)
Charge-offs	0	0	0	(5)
Ending balance	5,155	6,183	5,155	6,183
Allowance for loan loss	449	830	449	830
Defaults	0	0	$ 0	0
Number of payment deferral \| payment_deferral			28
Outstanding loans modified due to COVID-19	69,997		$ 69,997
Commercial and Industrial
TDR activity by loan category [Roll Forward]
Beginning balance	4	57	9	75
Additions	0	0	0	0
Principal (reductions) additions	(4)	(19)	(9)	(37)
Charge-offs	0	0	0	0
Ending balance	0	38	0	38
Allowance for loan loss	0	0	0	0
Defaults	0	0	$ 0	0
Commercial and Industrial \| Commercial and business loans
Troubled Debt Restructurings
Number of Contracts \| contract			0
Pre- Modification Outstanding Recorded Investment			$ 0
Post- Modification Outstanding Recorded Investment			$ 0
Number of Contracts which Defaulted During Period \| contract
Recorded Investment on Defaulted TDRs			$ 0
Commercial and Industrial \| Government program loans
Troubled Debt Restructurings
Number of Contracts \| contract			0
Pre- Modification Outstanding Recorded Investment			$ 0
Post- Modification Outstanding Recorded Investment			$ 0
Number of Contracts which Defaulted During Period \| contract			0
Recorded Investment on Defaulted TDRs			$ 0
Real estate mortgage \| Commercial real estate
Troubled Debt Restructurings
Number of Contracts \| contract			0
Pre- Modification Outstanding Recorded Investment			$ 0
Post- Modification Outstanding Recorded Investment			$ 0
Number of Contracts which Defaulted During Period \| contract			0
Recorded Investment on Defaulted TDRs			$ 0
TDR activity by loan category [Roll Forward]
Beginning balance	893	911	898	1,305
Additions	0	0	0	0
Principal (reductions) additions	(4)	(4)	(9)	(393)
Charge-offs	0	0	0	(5)
Ending balance	889	907	889	907
Allowance for loan loss	175	347	175	347
Defaults	0	0	$ 0	0
Real estate mortgage \| Residential mortgages
Troubled Debt Restructurings
Number of Contracts \| contract			0
Pre- Modification Outstanding Recorded Investment			$ 0
Post- Modification Outstanding Recorded Investment			$ 0
Number of Contracts which Defaulted During Period \| contract			0
Recorded Investment on Defaulted TDRs			$ 0
TDR activity by loan category [Roll Forward]
Beginning balance	1,048	1,857	1,060	2,029
Additions	0	0	0	0
Principal (reductions) additions	(10)	(16)	(22)	(188)
Charge-offs	0	0	0
Ending balance	1,038	1,841	1,038	1,841
Allowance for loan loss	31	32	31	32
Defaults	0	0	$ 0	0
Real estate mortgage \| Home improvement and home equity loans
Troubled Debt Restructurings
Number of Contracts \| contract			0
Pre- Modification Outstanding Recorded Investment			$ 0
Post- Modification Outstanding Recorded Investment			$ 0
Number of Contracts which Defaulted During Period \| contract			0
Recorded Investment on Defaulted TDRs			$ 0
TDR activity by loan category [Roll Forward]
Beginning balance	0	0	0	0
Additions	0	0	0	0
Principal (reductions) additions	0	0	0	0
Charge-offs	0	0	0	0
Ending balance	0	0	0	0
Allowance for loan loss	0	0	0	0
Defaults	0	0	$ 0	0
Real estate construction and development
Troubled Debt Restructurings
Number of Contracts \| contract			0
Pre- Modification Outstanding Recorded Investment			$ 0
Post- Modification Outstanding Recorded Investment			$ 0
Number of Contracts which Defaulted During Period \| contract			0
Recorded Investment on Defaulted TDRs			$ 0
TDR activity by loan category [Roll Forward]
Beginning balance	2,587	2,804	2,654	2,838
Additions	0	0	0	0
Principal (reductions) additions	(51)	(66)	(118)	(100)
Charge-offs	0	0	0	0
Ending balance	2,536	2,738	2,536	2,738
Allowance for loan loss	0	0	0	0
Defaults	0	0	$ 0	0
Agricultural loans
Troubled Debt Restructurings
Number of Contracts \| contract			1
Pre- Modification Outstanding Recorded Investment			$ 179
Post- Modification Outstanding Recorded Investment			$ 179
Number of Contracts which Defaulted During Period \| contract			0
Recorded Investment on Defaulted TDRs			$ 0
TDR activity by loan category [Roll Forward]
Beginning balance	721	711	566	812
Additions	0	0	179	0
Principal (reductions) additions	(29)	(52)	(53)	(153)
Charge-offs	0	0	0	0
Ending balance	692	659	692	659
Allowance for loan loss	243	451	243	451
Defaults	0	0	0	0
Installment and student loans
TDR activity by loan category [Roll Forward]
Beginning balance	0	0	0	0
Additions	0	0	0	0
Principal (reductions) additions	0	0	0	0
Charge-offs	0	0	0	0
Ending balance	0	0	0	0
Allowance for loan loss	0	0	0	0
Defaults	$ 0	$ 0	$ 0	$ 0
Installment and student loans \| Installment and student loans
Troubled Debt Restructurings
Number of Contracts \| contract			0
Pre- Modification Outstanding Recorded Investment			$ 0
Post- Modification Outstanding Recorded Investment			$ 0
Number of Contracts which Defaulted During Period \| contract			0
Recorded Investment on Defaulted TDRs			$ 0
Installment and student loans \| Overdraft protection lines
Troubled Debt Restructurings
Number of Contracts \| contract			0
Pre- Modification Outstanding Recorded Investment			$ 0
Post- Modification Outstanding Recorded Investment			$ 0
Number of Contracts which Defaulted During Period \| contract			0
Recorded Investment on Defaulted TDRs			$ 0

Financing Receivable, After Allowance For Credit Loss, Modified Due To COVID-19

No definition available.

Name:	ubfo_FinancingReceivableAfterAllowanceForCreditLossModifiedDueToCOVID19
Namespace Prefix:	ubfo_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Allowance for loan losses of the outstanding recorded investment related to financing receivables that have been modified by troubled debt restructurings.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Number Of Payment Deferrals

+ References

No definition available.

+ Details

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

- Definition

Number of financing receivables that have been restructured.

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

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

+ References

No definition available.

+ Details

Name:	ubfo_TroubledDebtRestructuringsAbstract
Namespace Prefix:	ubfo_
Data Type:	xbrli:stringItemType
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

No definition available.

+ Details

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

- Definition

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

+ References

+ Details

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

- Definition

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

+ References

+ Details

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

- Definition

Amount of financing receivable modified as troubled debt restructuring.

+ References

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=121611835&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

+ Details

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

- Definition

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

+ References

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=121611835&loc=SL6953682-111524

+ Details

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

- Definition

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

+ References

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=121611835&loc=SL6953682-111524

+ Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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

- Details

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