ACQUISITIONS - PRO FORMA INFORMATION (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Business Combinations [Abstract]
Pro forma revenues	$ 2,013.3	$ 2,037.5	$ 5,924.4	$ 5,916.3
Pro forma net income attributable to American Tower Corporation common stockholders	$ 464.3	$ 486.6	$ 1,324.7	$ 1,274.6
Pro forma net income per common share amounts:
Basic net income attributable to American Tower Corporation common stockholders (in dollars per share)	$ 1.05	$ 1.10	$ 2.99	$ 2.88
Diluted net income attributable to American Tower Corporation common stockholders (in dollars per share)	$ 1.04	$ 1.09	$ 2.97	$ 2.86