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Short-Term Borrowings - Additional Information (Detail) (USD $)
12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Short-term Debt [Line Items]    
Federal funds purchased   $ 0us-gaap_FederalFundsPurchased
Securities sold under repurchase agreements 436,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase 1,231,000us-gaap_SecuritiesSoldUnderAgreementsToRepurchase
Available fund lines from correspondent banks 18,800,000usbi_AvailableFundLinesFromCorrespondentBanks $ 18,800,000usbi_AvailableFundLinesFromCorrespondentBanks
Minimum [Member]    
Short-term Debt [Line Items]    
Maturity period of federal funds 1 day  
Maximum [Member]    
Short-term Debt [Line Items]    
Maturity period of federal funds 4 days