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Guarantees, Commitments and Contingencies - Additional Information (Detail) (USD $)	3 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Dec. 31, 2014
Commitments And Contingencies Disclosure [Abstract]
Credit losses associated with derivative contracts	$ 0usbi_CreditLossesAssociatedWithDerivativeContracts	$ 0usbi_CreditLossesAssociatedWithDerivativeContracts
Outstanding commitments to purchase securities	0usbi_SecuritiesCommittedToPurchaseAmount		0usbi_SecuritiesCommittedToPurchaseAmount
Outstanding commitments to sell securities	$ 0usbi_SecuritiesCommittedToSellAmount		$ 0usbi_SecuritiesCommittedToSellAmount

X

- Definition

Credit losses associated with derivative contracts.

+ References

No definition available.

+ Details

Name:	usbi_CreditLossesAssociatedWithDerivativeContracts
Namespace Prefix:	usbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

X

- Definition

Securities committed to purchase amount.

+ References

No definition available.

+ Details

Name:	usbi_SecuritiesCommittedToPurchaseAmount
Namespace Prefix:	usbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Definition

Securities committed to sell amount.

+ References

No definition available.

+ Details

Name:	usbi_SecuritiesCommittedToSellAmount
Namespace Prefix:	usbi_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

X

- Details

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