XML 23 R1.htm IDEA: XBRL DOCUMENT v3.10.0.1
Document and Entity Information - USD ($)
 12 Months Ended
Dec. 31, 2018
Feb. 19, 2019
Jun. 30, 2018
Document and Entity Information      
Entity Registrant Name AFFILIATED MANAGERS GROUP, INC.    
Entity Central Index Key 0001004434    
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
Amendment Flag false    
Current Fiscal Year End Date --12-31    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Filer Category Large Accelerated Filer    
Entity Small Business false    
Entity Emerging Growth Company false    
Entity Shell Company false    
Entity Public Float     $ 7,886,822,780
Entity Common Stock, Shares Outstanding   52,048,705  
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus FY