| Cover | Dec. 28, 2020 |
| Cover [Abstract] | |
| Document Type | 8-K |
| Amendment Flag | false |
| Document Period End Date | Dec. 28, 2020 |
| Entity File Number | 001-38604 |
| Entity Registrant Name | FOCUS FINANCIAL PARTNERS INC. |
| Entity Central Index Key | 0001651052 |
| Entity Tax Identification Number | 47-4780811 |
| Entity Incorporation, State or Country Code | DE |
| Entity Address, Address Line One | 875 Third Avenue |
| Entity Address, Address Line Two | 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 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Class A common stock, par value $0.01 per share |
| Trading Symbol | FOCS |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
