EARNINGS PER COMMON SHARE (Schedule of Earnings Per Basic And Diluted by Common Class) (Details) - USD ($) $ / shares in Units, shares in Thousands, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Earnings Per Share [Abstract]
Net income attributable to American Tower Corporation stockholders	$ 238.5	$ 317.3	$ 367.0	$ 316.1	$ 229.2	$ 264.5	$ 187.6	$ 275.2	$ 1,238.9	$ 956.4	$ 685.1
Dividends on preferred stock	(18.9)	(18.9)	(22.8)	(26.8)	(26.8)	(26.8)	(26.8)	(26.8)	(87.4)	(107.1)	(90.2)
NET INCOME ATTRIBUTABLE TO AMERICAN TOWER CORPORATION COMMON STOCKHOLDERS	$ 219.6	$ 298.4	$ 344.2	$ 289.3	$ 202.4	$ 237.7	$ 160.8	$ 248.4	$ 1,151.5	$ 849.3	$ 594.9
Basic weighted average common shares outstanding									428,181	425,143	418,907
Dilutive securities (shares)									3,507	4,140	4,108
Diluted weighted average common shares outstanding									431,688	429,283	423,015
Basic net income attributable to American Tower Corporation common stockholders (in dollars per share)	$ 0.51	$ 0.70	$ 0.81	$ 0.68	$ 0.48	$ 0.56	$ 0.38	$ 0.59	$ 2.69	$ 2.00	$ 1.42
Diluted net income attributable to American Tower Corporation common stockholders (in dollars per share)	$ 0.51	$ 0.69	$ 0.80	$ 0.67	$ 0.47	$ 0.55	$ 0.37	$ 0.58	$ 2.67	$ 1.98	$ 1.41