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Leases - Narrative (Details)	9 Months Ended
	Sep. 30, 2021 contract
Leases [Abstract]
Operating lease, number of contracts	12
Financing lease, number of contracts	0

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- Definition

Lessee, Financing Lease, Number Of Contracts

+ References

No definition available.

+ Details

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- Definition

Lessee, Operating Lease, Number Of Contracts

+ References

No definition available.

+ Details

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Namespace Prefix:	ubfo_
Data Type:	xbrli:integerItemType
Balance Type:	na
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- References

No definition available.

+ Details

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