| Document and Entity Information | Aug. 03, 2020 |
| Document And Entity Information [Line Items] | |
| Amendment Flag | false |
| Entity Central Index Key | 0001004434 |
| Document Type | 8-K |
| Document Period End Date | Aug. 3, 2020 |
| Entity Registrant Name | AFFILIATED MANAGERS GROUP, INC. |
| Entity Incorporation State 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 |
| Common Stock [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | Common Stock ($0.01 par value) |
| Trading Symbol | AMG |
| Security Exchange Name | NYSE |
| Junior Notes [Member] | |
| Document And Entity Information [Line Items] | |
| Security 12b Title | 5.875% Junior Subordinated Notes due 2059 |
| Trading Symbol | MGR |
| Security Exchange Name | NYSE |
