v3.25.2
Document and Entity Information - shares
6 Months Ended
Jun. 30, 2025
Aug. 01, 2025
Cover [Abstract]    
Document Type 10-Q  
Amendment Flag false  
Document Period End Date Jun. 30, 2025  
Document Fiscal Year Focus 2025  
Document Fiscal Period Focus Q2  
Entity Registrant Name FIRST US BANCSHARES INC  
Entity Central Index Key 0000717806  
Current Fiscal Year End Date --12-31  
Entity Filer Category Non-accelerated Filer  
Entity Small Business true  
Entity Emerging Growth Company false  
Entity Common Stock, Shares Outstanding   5,759,659
Entity Current Reporting Status Yes  
Entity Interactive Data Current Yes  
Entity Shell Company false  
Document Quarterly Report true  
Document Transition Report false  
Entity File Number 000-14549  
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  
Title of 12(b) Security Common Stock, $0.01 par value  
Trading Symbol FUSB  
Security Exchange Name NASDAQ