v3.19.1
Document and Entity Information - USD ($)
12 Months Ended
Dec. 31, 2018
Mar. 25, 2019
Jun. 30, 2018
Entity Registrant Name Focus Financial Partners Inc.    
Entity Central Index Key 0001651052    
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer No    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Non-accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company true    
Entity Ex Transition Period true    
Entity Shell Company false    
Entity Public Float     $ 0
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY    
Class A common stock      
Entity Common Stock, Shares Outstanding   46,675,183  
Class B common stock      
Entity Common Stock, Shares Outstanding   22,568,831