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Cover Page - USD ($)
 12 Months Ended
Dec. 31, 2021
Feb. 16, 2022
Jun. 30, 2021
Cover [Abstract]      
Document Type 10-K    
Document Annual Report true    
Document Period End Date Dec. 31, 2021    
Current Fiscal Year End Date --12-31    
Document Transition Report false    
Entity File Number 001-13459    
Entity Registrant Name AFFILIATED MANAGERS GROUP, INC.    
Entity Incorporation, State or Country Code DE    
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    
Entity Information [Line Items]      
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
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    
ICFR Auditor Attestation Flag true    
Entity Shell Company false    
Entity Public Float     $ 6,336,812,433
Entity Common Stock, Shares Outstanding (in shares)   39,778,543  
Documents Incorporated by Reference Portions of the definitive proxy statement to be filed with the Securities and Exchange Commission within 120 days after December 31, 2021, and delivered to stockholders in connection with the registrant’s annual meeting of stockholders, are incorporated by reference into Part III.    
Entity Central Index Key 0001004434    
Amendment Flag false    
Document Fiscal Year Focus 2021    
Document Fiscal Period Focus FY    
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    
4.750% 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    
2061 Junior Subordinated Notes      
Entity Information [Line Items]      
Title of 12(b) Security 4.200% Junior Subordinated Notes due 2061    
Trading Symbol MGRD    
Security Exchange Name NYSE