| Cover page | Feb. 08, 2021 |
| Entity Information [Line Items] | |
| Document Type | 8-K |
| Document Period End Date | Feb. 8, 2021 |
| Entity Registrant Name | AFFILIATED MANAGERS GROUP, INC. |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-13459 |
| Entity Tax Identification Number | 04-3218510 |
| Entity Address, Address Line One | 777 South Flagler Drive |
| Entity Address, City or Town | West Palm Beach |
| Entity Address, State or Province | FL |
| Entity Address, Postal Zip Code | 33401 |
| City Area Code | 800 |
| Local Phone Number | 345-1100 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Entity Emerging Growth Company | false |
| Entity Central Index Key | 0001004434 |
| Amendment Flag | false |
| Common Stock ($0.01 par value) | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | Common Stock ($0.01 par value) |
| Trading Symbol | AMG |
| Security Exchange Name | NYSE |
| 5.875% Junior Subordinated Notes due 2059 | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | 5.875% Junior Subordinated Notes due 2059 |
| Trading Symbol | MGR |
| Security Exchange Name | NYSE |
| Four Point Seventy Five Junior Subordinated Notes Due 2060 | |
| Entity Information [Line Items] | |
| Title of 12(b) Security | 4.750% Junior Subordinated Notes due 2060 |
| Trading Symbol | MGRB |
| Security Exchange Name | NYSE |
