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, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Earnings Per Share [Abstract]
Net income attributable to american tower corporation stockholders	$ 562.7	$ 498.6	$ 429.1	$ 397.4	$ 277.6	$ 366.9	$ 306.7	$ 285.2	$ 1,887.8	$ 1,236.4	$ 1,238.9
Dividends on preferred stock					0.0	0.0	0.0	(9.4)	0.0	(9.4)	(87.4)
Net income attributable to american tower corporation common stockholders	$ 562.7	$ 498.6	$ 429.1	$ 397.4	$ 277.6	$ 366.9	$ 306.7	$ 275.8	$ 1,887.8	$ 1,227.0	$ 1,151.5
Basic weighted average common shares outstanding (in shares)									442,319	439,606	428,181
Dilutive securities (shares)									3,201	3,354	3,507
Diluted weighted average common shares outstanding (in shares)									445,520	442,960	431,688
Basic net income attributable to american tower corporation common stockholders (in dollars per share)	$ 1.27	$ 1.13	$ 0.97	$ 0.90	$ 0.63	$ 0.83	$ 0.69	$ 0.63	$ 4.27	$ 2.79	$ 2.69
Diluted net income attributable to american tower corporation common stockholders (in dollars per share)	$ 1.26	$ 1.12	$ 0.96	$ 0.89	$ 0.62	$ 0.83	$ 0.69	$ 0.63	$ 4.24	$ 2.77	$ 2.67