Student Loans - Credit Quality Indicators (Details) $ in Thousands	Sep. 30, 2019 USD ($) loan	Dec. 31, 2018 USD ($) loan
Financing Receivable, Credit Quality Indicator [Line Items]
Accrued interest	$ 10,522	$ 8,341
Student loan
Financing Receivable, Credit Quality Indicator [Line Items]
Number of loans \| loan	1,560	1,674
Loans outstanding	$ 65,655	$ 68,221
Accrued interest	$ 8,176	$ 5,984
Student loan \| School
Financing Receivable, Credit Quality Indicator [Line Items]
Number of loans \| loan	586	1,056
Loans outstanding	$ 23,477	$ 42,852
Accrued interest	$ 4,049	$ 5,494
Student loan \| Grace
Financing Receivable, Credit Quality Indicator [Line Items]
Number of loans \| loan	404	23
Loans outstanding	$ 18,619	$ 562
Accrued interest	$ 3,765	$ 81
Student loan \| Repayment
Financing Receivable, Credit Quality Indicator [Line Items]
Number of loans \| loan	328	366
Loans outstanding	$ 14,421	$ 15,526
Accrued interest	$ 120	$ 118
Student loan \| Deferment
Financing Receivable, Credit Quality Indicator [Line Items]
Number of loans \| loan	73	48
Loans outstanding	$ 2,158	$ 1,945
Accrued interest	$ 108	$ 79
Student loan \| Forbearance
Financing Receivable, Credit Quality Indicator [Line Items]
Number of loans \| loan	169	181
Loans outstanding	$ 6,980	$ 7,336
Accrued interest	$ 134	$ 212

Number of Loans

No definition available.

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

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

- Definition

Carrying amount as of the balance sheet date of interest earned but not received. Also called accrued interest or accrued interest receivable.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 210
-SubTopic 10
-Section S99
-Paragraph 1
-Subparagraph (SX 210.5-02.8)
-URI http://asc.fasb.org/extlink&oid=6877327&loc=d3e13212-122682

+ Details

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

- Definition

Amount, before allowance for credit loss, of financing receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 20
-Section 50
-Paragraph 5
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=119407570&loc=SL82919230-210447

+ Details

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

- Details

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

- Details

Name:	ubfo_LoanStatusAxis=ubfo_SchoolMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ubfo_LoanStatusAxis=ubfo_GraceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ubfo_LoanStatusAxis=ubfo_RepaymentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ubfo_LoanStatusAxis=ubfo_DefermentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	ubfo_LoanStatusAxis=ubfo_ForbearanceMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: