SPECIAL (GAINS) AND CHARGES - Restructuring Activities, Acquisition and Integration Related Costs (Details) 10Q $ in Millions	3 Months Ended		6 Months Ended
	Mar. 31, 2018 USD ($)	Mar. 31, 2017 USD ($)	Sep. 30, 2017 item	Dec. 31, 2017 USD ($)
Restructuring
Restructuring liability	$ 34.0			$ 41.5
Cash payments	7.9
Acquisition, integrated and other related costs
Property, Plant and Equipment, Net	3,779.9			$ 3,707.1
Special (gains) and charges
Restructuring
Restructuring charges incurred, pre-tax	0.3	$ (0.3)
Restructuring charges incurred, after tax	0.3	(0.2)
Acquisition, integrated and other related costs
Acquisition and integration costs	0.5	6.3
Acquisition and integration costs, after tax	0.3	4.2
Other	25.2	0.2
Other, Ecolab Foundation, pre-tax	25.0
Other, Ecolab Foundation, after tax	$ 18.9
Cost of sales
Acquisition, integrated and other related costs
Acquisition and integration costs		$ 1.5
2017 Restructuring Plan
Restructuring
Number of positions expected to be eliminated | item			570