Document and Entity Information - USD ($)
$ in Billions
12 Months Ended
Dec. 31, 2018
Feb. 20, 2019
Jun. 30, 2018
Document And Entity Information [Abstract]      
Entity Registrant Name AMERICAN TOWER CORP /MA/    
Entity Central Index Key 0001053507    
Current Fiscal Year End Date --12-31    
Entity Filer Category Large Accelerated Filer    
Document Type 10-K    
Document Period End Date Dec. 31, 2018    
Document Fiscal Year Focus 2018    
Document Fiscal Period Focus (Q1,Q2,Q3,FY) Q4    
Amendment Flag false    
Entity Common Stock, Shares Outstanding   441,134,906  
Entity Emerging Growth Company false    
Entity Small Business false    
Entity Shell Company false    
Entity Well-known Seasoned Issuer Yes    
Entity Voluntary Filers No    
Entity Current Reporting Status Yes    
Entity Public Float     $ 63.1