NEW ACCOUNTING PRONOUNCEMENTS - Revenue Recognition and Pension Standards (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Dec. 31, 2016
Net sales	$ 3,470.9	$ 3,162.4	$ 13,835.9
Cost of sales (including special charges)	2,082.1	1,851.8
Selling, general and administrative expenses	1,008.5	947.2
Special (gains) and charges	26.0	6.2
Operating income	354.3	357.2	1,950.1
Other (income)/expense	(19.4)	(16.8)
Interest expense, net	56.4	62.5
Income before income taxes	317.3	311.5
Provision for income taxes	69.1	54.2
Net income including noncontrolling interest	248.2	257.3	1,518.6	$ 1,246.5
Net income attributable to noncontrolling interest	0.9	3.3
Net income attributable to Ecolab	$ 247.3	$ 254.0
Earnings attributable to Ecolab per common share
Basic (in dollars per share)	$ 0.86	$ 0.87
Diluted (in dollars per share)	$ 0.84	$ 0.86
Previously Reported
Net sales		$ 3,161.6
Net sales		3,161.6	13,838.3
Cost of sales (including special charges)		1,691.5
Cost of sales (including special charges)		1,691.5
Selling, general and administrative expenses		1,090.6
Special (gains) and charges		6.2
Operating income		373.3	2,019.8
Interest expense, net		62.5
Income before income taxes		310.8
Provision for income taxes		54.0
Net income including noncontrolling interest		256.8
Net income attributable to noncontrolling interest		3.3
Net income attributable to Ecolab		$ 253.5
Earnings attributable to Ecolab per common share
Basic (in dollars per share)		$ 0.87
Diluted (in dollars per share)		$ 0.86
Adjustment | Revenue Standard Adoption
Net sales		$ (3,161.6)
Net sales		0.8	(2.4)
Cost of sales (including special charges)		(1,691.5)
Cost of sales (including special charges)		159.2
Selling, general and administrative expenses		(159.1)
Operating income		0.7	(2.4)
Income before income taxes		0.7
Provision for income taxes		0.2
Net income including noncontrolling interest		0.5
Net income attributable to Ecolab		0.5
Adjustment | Pension Standard Adoption
Cost of sales (including special charges)		1.1
Selling, general and administrative expenses		15.7
Operating income		(16.8)	$ (67.3)
Other (income)/expense		(16.8)
Product and equipment
Net sales	$ 2,847.2	2,604.4
Cost of sales (including special charges)	1,696.6	1,500.7
Product and equipment | Adjustment | Revenue Standard Adoption
Net sales		2,604.4
Cost of sales (including special charges)		1,499.8
Product and equipment | Adjustment | Pension Standard Adoption
Cost of sales (including special charges)		0.9
Service and lease
Net sales	623.7	558.0
Cost of sales (including special charges)	$ 385.5	351.1
Service and lease | Adjustment | Revenue Standard Adoption
Net sales		558.0
Cost of sales (including special charges)		350.9
Service and lease | Adjustment | Pension Standard Adoption
Cost of sales (including special charges)		$ 0.2