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Short-term Borrowings/Other Borrowings (Details) (USD $)
Mar. 31, 2015
Dec. 31, 2014
Short-term Debt [Line Items]    
Short-term debt outstanding $ 0us-gaap_FederalHomeLoanBankAdvancesCurrent $ 0us-gaap_FederalHomeLoanBankAdvancesCurrent
Federal Reserve Bank of San Francisco    
Short-term Debt [Line Items]    
Unused borrowing lines 313,928,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_FederalReserveBankAdvancesMember
 
Short-term Debt   286,993,000us-gaap_ShortTermBorrowings
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_FederalReserveBankAdvancesMember
Qualifying loans pledged as collateral for borrowing lines 441,949,000us-gaap_LoansPledgedAsCollateral
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_FederalReserveBankAdvancesMember
406,358,000us-gaap_LoansPledgedAsCollateral
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_FederalReserveBankAdvancesMember
Federal Home Loan Bank (FHLB)    
Short-term Debt [Line Items]    
Short-term Debt 5,590,000us-gaap_ShortTermBorrowings
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
5,814,000us-gaap_ShortTermBorrowings
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
Investment securities pledged as collateral 5,868,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
6,106,000us-gaap_SecurityOwnedAndPledgedAsCollateralFairValue
/ us-gaap_ShortTermDebtTypeAxis
= us-gaap_FederalHomeLoanBankAdvancesMember
Pacific Coast Bankers Bank    
Short-term Debt [Line Items]    
Unused borrowing lines $ 10,000,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_ShortTermDebtTypeAxis
= ubfo_PacificCoastBankersBankMember
$ 10,000,000us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity
/ us-gaap_ShortTermDebtTypeAxis
= ubfo_PacificCoastBankersBankMember