Document and Entity Information - shares	3 Months Ended
	Mar. 31, 2020	May 04, 2020
Document Type	10-Q
Document Quarterly Report	true
Document Transition Report	false
Document Period End Date	Mar. 31, 2020
Entity File Number	001-38604
Entity Registrant Name	Focus Financial Partners Inc.
Entity Incorporation, State or Country Code	DE
Entity Tax Identification Number	47-4780811
Entity Address, Address Line One	875 Third Avenue, 28th Floor
Entity Address, City or Town	New York
Entity Address, State or Province	NY
Entity Address, Postal Zip Code	10022
City Area Code	646
Local Phone Number	519-2456
Title of 12(b) Security	Class A common stock, par value$0.01 per share
Trading Symbol	FOCS
Security Exchange Name	NASDAQ
Entity Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2020
Document Fiscal Period Focus	Q1
Entity Central Index Key	0001651052
Amendment Flag	false
Class A common stock
Entity Common Stock, Shares Outstanding		47,806,195
Class B common stock
Entity Common Stock, Shares Outstanding		21,759,379