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, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Share-Based Compensation
Excess tax benefits, share-based compensation									$ 28.1	$ 39.7
New accounting guidance, cumulative effect									(43.6)	1.9	$ (29.3)
Computations of the basic and diluted earnings attributable to Ecolab per share amounts
Net income attributable to Ecolab	$ 395.1	$ 435.4	$ 351.3	$ 247.3	$ 562.6	$ 393.2	$ 294.8	$ 254.0	$ 1,429.1	$ 1,504.6	$ 1,229.0
Weighted-average common shares outstanding
Basic (in shares)	288.0	288.8	288.8	288.6	289.1	289.0	289.8	290.6	288.6	289.6	292.5
Effect of dilutive stock options and units (in shares)									4.2	4.4	4.2
Diluted (in shares)	292.2	293.4	293.3	292.7	293.6	293.4	294.1	295.0	292.8	294.0	296.7
Earnings attributable to Ecolab per common share
Basic (in dollars per share)	$ 1.37	$ 1.51	$ 1.22	$ 0.86	$ 1.95	$ 1.36	$ 1.02	$ 0.87	$ 4.95	$ 5.20	$ 4.20
Diluted (in dollars per share)	$ 1.35	$ 1.48	$ 1.20	$ 0.84	$ 1.92	$ 1.34	$ 1.00	$ 0.86	$ 4.88	$ 5.12	$ 4.14
Anti-dilutive stock options, units and awards excluded from computation of earnings per share (in shares)									2.9	3.4	3.6