Document and Entity Information - shares
6 Months Ended
Jun. 30, 2019
Jul. 30, 2019
Cover page.    
Document Type 10-Q  
Document Quarterly Report true  
Document Period End Date Jun. 30, 2019  
Document Transition Report false  
Entity File Number 001-13459  
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   50,639,554
Entity Central Index Key 0001004434  
Amendment Flag false  
Current Fiscal Year End Date --12-31  
Document Fiscal Year Focus 2019  
Document Fiscal Period Focus Q2  
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