| Cover | Feb. 25, 2021 |
| Cover [Abstract] | |
| Document Type | 8-K/A |
| Amendment Flag | true |
| Amendment Description | The purpose of this Amendment No. 1 ("Amendment No. 1") to the Current Report on Form 8-K of Focus Financial Partners Inc., filed with the Securities and Exchange Commission on March 2, 2021 (the "Original Form 8-K"), is to replace Exhibit 5.1, Opinion of Vinson & Elkins L.L.P., of the Original Form 8-K with Exhibit 5.1 attached to this Amendment No. 1. Other than as described herein, this Amendment No. 1 does not amend any other information previously filed in the Original Form 8-K and does not otherwise reflect events occurring after the original filing of the Original Form 8-K. |
| Document Period End Date | Feb. 25, 2021 |
| 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 |
