SPECIAL (GAINS) AND CHARGES - Restructuring and Non-Restructuring Activity (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018
Restructuring
Recorded expense	$ 40.5		$ 104.6
Net cash payments	(22.8)		(35.2)
Non-cash charges	(0.3)		(5.0)
Effect of foreign currency translation	0.1		(0.5)
Restructuring liability	81.4		63.9
Other restructuring information
Special (gains) and charges	40.3	$ 26.0
Non-restructuring Special (Gains) and Charges
Property, Plant and Equipment, Net	3,878.6		3,836.0
Special (gains) and charges
Other restructuring information
Restructuring charges incurred, pre-tax	37.1	0.3
Upstream energy spin-off	4.3
Upstream energy spin-off, net of tax	3.3
Special (gains) and charges	40.3	26.0
Business combination advisory and legal fees, pre tax	2.5	0.5
Business combination advisory and legal fees, after tax	1.8	0.3
Other special gains and charges	(3.6)	25.2
Other special gains and charges, after-tax	(4.3)	18.9
Other, Ecolab Foundation, pre-tax		25.0
Other, Ecolab Foundation, after tax		18.9
Cost of sales
Other restructuring information
Restructuring charges incurred, pre-tax	3.4
Special (gains) and charges	3.6
Business combination advisory and legal fees, pre tax	0.2
Interest expense
Other restructuring information
Special (gains) and charges	0.2
Special (gains) and charges, net of tax	0.1
Employee termination costs
Restructuring
Recorded expense	29.4		94.1
Net cash payments	(20.0)		(32.8)
Effect of foreign currency translation	0.1		(0.5)
Restructuring liability	70.3		60.8
Asset disposals
Restructuring
Recorded expense	0.3		5.0
Non-cash charges	(0.3)		(5.0)
Other
Restructuring
Recorded expense	10.8		5.5
Net cash payments	(2.8)		(2.4)
Restructuring liability	11.1		3.1
2018 Restructuring Plan
Other restructuring information
Restructuring charge expected to be incurred, pre-tax	260.0
Restructuring charge expected to be incurred, after tax	190.0
2018 Restructuring Plan | Special (gains) and charges
Restructuring
Restructuring liability	81.4
Other restructuring information
Restructuring charges incurred to date, pre-tax	145.1
Restructuring charges incurred to date, after-tax	110.0
Restructuring charges incurred, pre-tax	40.5
Restructuring charges incurred, after tax	30.4
Prior Year Plans
Restructuring
Cash payments	(1.4)
Restructuring liability	13.1		$ 14.9
Other restructuring information
Restructuring charges incurred, pre-tax		0.3
Restructuring charges incurred, after tax		$ 0.3
Cash payments	$ 1.4