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Document and Entity Information - USD ($)
$ in Millions
 12 Months Ended
Dec. 31, 2019
Feb. 18, 2020
Document and Entity Information [Abstract]    
Entity Registrant Name First Business Financial Services, Inc.  
Entity Central Index Key 0001521951  
Current Fiscal Year End Date --12-31  
Document Type 10-K  
Document Period End Date Dec. 31, 2019  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus FY  
Amendment Flag false  
Entity Emerging Growth Company false  
Entity Small Business true  
Entity Interactive Data Current Yes  
Entity Well-known Seasoned Issuer No  
Entity Shell Company false  
Entity Voluntary Filers No  
Entity Current Reporting Status Yes  
Entity Filer Category Accelerated Filer  
Entity Public Float $ 203.9  
Entity Common Stock, Shares Outstanding   8,513,390