SELECTED QUARTERLY FINANCIAL DATA (UNAUDITED) (Details) - USD ($) $ / shares in Units, $ 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
Selected Quarterly Financial Information [Abstract]
Operating revenues	$ 1,923.7	$ 1,953.6	$ 1,889.6	$ 1,813.4	$ 2,131.9	$ 1,785.5	$ 1,780.9	$ 1,741.8	$ 7,580.3	$ 7,440.1	$ 6,663.9
Cost of operations	550.2	559.9	563.3	543.4	540.9	556.7	560.3	519.9	2,216.8	2,177.8
Operating income	661.3	728.3	683.9	614.9	388.9	567.2	546.0	402.9	2,688.4	1,905.0	1,998.4
Net income	569.4	505.3	434.3	407.6	292.7	377.3	314.4	280.3	1,916.6	1,264.7	1,225.4
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 net income per common share (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 per common share (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