Cover Page - shares	9 Months Ended
	Sep. 30, 2021	Nov. 03, 2021
Cover [Abstract]
Document Type	10-Q
Document Quarterly Report	true
Document Period End Date	Sep. 30, 2021
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 Current Reporting Status	Yes
Entity Interactive Data Current	Yes
Entity Filer Category	Large Accelerated Filer
Entity Small Business	false
Entity Emerging Growth Company	false
Entity Shell Company	false
Entity Common Stock, Shares Outstanding (in shares)		40,651,314
Entity Central Index Key	0001004434
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2021
Document Fiscal Period Focus	Q3
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
2059 Junior Subordinated Notes
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
4.200% Junior Subordinated Notes due 2061
Entity Information [Line Items]
Title of 12(b) Security	4.200% Junior Subordinated Notes due 2061
Trading Symbol	MGRD
Security Exchange Name	NYSE