XML 15 R1.htm IDEA: XBRL DOCUMENT v3.24.0.1
Cover - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2023
Feb. 26, 2024
Jun. 30, 2023
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2023    
Document Transition Report false    
Entity File Number 001-34095    
Entity Registrant Name FIRST BUSINESS FINANCIAL SERVICES, INC.    
Entity Central Index Key 0001521951    
Current Fiscal Year End Date --12-31    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
Amendment Flag false    
Entity Incorporation, State or Country Code WI    
Entity Tax Identification Number 39-1576570    
Entity Address, Address Line One 401 Charmany Drive    
Entity Address, City or Town Madison    
Entity Address, State or Province WI    
Entity Address, Postal Zip Code 53719    
City Area Code 608    
Local Phone Number 238-8008    
Title of 12(b) Security Common Stock, $0.01 par value    
Trading Symbol FBIZ    
Security Exchange Name NASDAQ    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Interactive Data Current Yes    
Entity Filer Category Accelerated Filer    
Entity Small Business true    
Entity Emerging Growth Company false    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 245.2
Entity Common Stock, Shares Outstanding   8,306,543  
Documents Incorporated by Reference    
Document Financial Statement Error Correction [Flag] false