SIGNIFICANT ACCOUNTING POLICIES - Earnings Per Common Share (Details) - USD ($) $ / shares in Units, shares in Millions, $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Computations of the basic and diluted earnings attributable to Ecolab per share amounts
Net income from continuing operations attributable to Ecolab	$ 300.3	$ 246.2	$ 128.9	$ 292.0	$ 394.7	$ 435.9	$ 343.4	$ 251.6	$ 967.4	$ 1,425.6	$ 1,250.3
Net (loss) income from discontinued operations, net of tax			(2,163.9)	(8.6)	34.9	28.3	25.2	44.9	(2,172.5)	133.3	178.8
Net (loss) income attributable to Ecolab	$ 300.3	$ 246.2	$ (2,035.0)	$ 283.4	$ 429.6	$ 464.2	$ 368.6	$ 296.5	$ (1,205.1)	$ 1,558.9	$ 1,429.1
Weighted-average common shares outstanding
Basic (in shares)	285.6	285.4	288.2	288.8	288.3	288.1	287.6	288.2	287.0	288.1	288.6
Effect of dilutive stock options and units (in shares)									3.3	4.4	4.2
Diluted (in shares)	288.7	288.4	291.5	292.6	292.6	292.8	292.1	292.3	290.3	292.5	292.8
Basic EPS
Continuing operations	$ 1.05	$ 0.86	$ 0.45	$ 1.01	$ 1.37	$ 1.51	$ 1.19	$ 0.87	$ 3.37	$ 4.95	$ 4.33
Discontinued operations			(7.51)	(0.03)	0.12	0.10	0.09	0.16	(7.57)	0.46	0.62
Earnings (loss) attributable to Ecolab	1.05	0.86	(7.06)	0.98	1.49	1.61	1.28	1.03	(4.20)	5.41	4.95
Diluted EPS
Continuing operations	1.04	0.85	0.44	1.00	1.35	1.49	1.18	0.86	3.33	4.87	4.27
Discontinued operations			(7.42)	(0.03)	0.12	0.10	0.09	0.15	(7.48)	0.46	0.61
Earnings (loss) attributable to Ecolab	$ 1.04	$ 0.85	$ (6.98)	$ 0.97	$ 1.47	$ 1.59	$ 1.26	$ 1.01	$ (4.15)	$ 5.33	$ 4.88
Anti-dilutive securities excluded from computation of earnings per share (in shares)									1.9	1.1	2.9