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Cover Page - USD ($)	12 Months Ended
Cover Page - USD ($)	Dec. 31, 2019	Feb. 25, 2020	Jun. 30, 2019
Cover page.
Document Type	10-K
Document Annual Report	true
Document Period End Date	Dec. 31, 2019
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
Entity Shell Company	false
Entity Public Float			$ 4,627,759,695
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, 2019, 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
Entity Common Stock, Shares Outstanding		47,867,765
Amendment Flag	false
Current Fiscal Year End Date	--12-31
Document Fiscal Year Focus	2019
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