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Document And Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2020
Mar. 12, 2021
Jun. 30, 2020
Cover [Abstract]      
Entity Registrant Name FIRST US BANCSHARES INC    
Document Type 10-K    
Trading Symbol FUSB    
Current Fiscal Year End Date --12-31    
Entity Common Stock, Shares Outstanding   6,213,641  
Entity Central Index Key 0000717806    
Entity Filer Category Non-accelerated Filer    
Document Period End Date Dec. 31, 2020    
Document Fiscal Year Focus 2020    
Document Fiscal Period Focus FY    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Shell Company false    
Entity Small Business true    
Entity Emerging Growth Company false    
Security Exchange Name NASDAQ    
Entity File Number 0-14549    
Entity Incorporation, State or Country Code DE    
Entity Tax Identification Number 63-0843362    
Entity Address, Address Line One 3291 U.S. Highway 280    
Entity Address, City or Town Birmingham    
Entity Address, State or Province AL    
Entity Address, Postal Zip Code 35243    
City Area Code 205    
Local Phone Number 582-1200    
Document Annual Report true    
Document Transition Report false    
ICFR Auditor Attestation Flag false    
Title of 12(b) Security Common Stock, par value $0.01 per share    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Public Float     $ 40,043,138
Documents Incorporated by Reference

Portions of the registrant’s definitive proxy statement for the 2021 Annual Meeting of Shareholders to be held on April 29, 2021 are incorporated by reference into Part III of this Annual Report on Form 10-K.

    
Amendment Flag false