NEW ACCOUNTING PRONOUNCEMENTS - Revenue Recognition and Pension Standards (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016
Net sales	$ 3,747.2	$ 3,564.5	$ 10,907.7	$ 10,186.9	$ 13,835.9
Cost of sales (including special charges)	2,200.2	2,064.6	6,438.4	5,961.8
Selling, general and administrative expenses	955.2	930.9	2,990.5	2,836.3
Special (gains) and charges	75.6	4.9	113.7	47.9
Operating income	516.2	564.1	1,365.1	1,340.9	1,950.1
Other (income)/expense	(21.0)	(16.9)	(60.0)	(50.5)
Interest expense, net	55.7	55.1	168.4	177.2
Income before income taxes	481.5	525.9	1,256.7	1,214.2
Provision for income taxes	43.2	129.3	216.6	264.0
Net income including noncontrolling interest	438.3	396.6	1,040.1	950.2	1,518.6	$ 1,246.5
Net income attributable to noncontrolling interest	2.9	3.4	6.1	8.2
Net income attributable to Ecolab	$ 435.4	$ 393.2	$ 1,034.0	$ 942.0
Earnings attributable to Ecolab per common share
Basic (in dollars per share)	$ 1.51	$ 1.36	$ 3.58	$ 3.25
Diluted (in dollars per share)	$ 1.48	$ 1.34	$ 3.53	$ 3.20
Previously Reported
Net sales		$ 3,563.3		$ 10,187.6
Net sales		3,563.3		10,187.6	13,838.3
Cost of sales (including special charges)		1,891.3		5,454.4
Cost of sales (including special charges)		1,891.3		5,454.4
Selling, general and administrative expenses		1,087.3		3,293.2
Special (gains) and charges		4.9		47.9
Operating income		579.8		1,392.1	2,019.8
Interest expense, net		55.1		177.2
Income before income taxes		524.7		1,214.9
Provision for income taxes		128.9		264.2
Net income including noncontrolling interest		395.8		950.7
Net income attributable to noncontrolling interest		3.4		8.2
Net income attributable to Ecolab		$ 392.4		$ 942.5
Earnings attributable to Ecolab per common share
Basic (in dollars per share)		$ 1.36		$ 3.25
Diluted (in dollars per share)		$ 1.34		$ 3.20
Adjustment | Revenue Standard Adoption
Net sales		$ (3,563.3)		$ (10,187.6)
Net sales		1.2		(0.7)	(2.4)
Cost of sales (including special charges)		(1,891.3)		(5,454.4)
Cost of sales (including special charges)		172.0		503.7
Selling, general and administrative expenses		(172.0)		(503.7)
Operating income		1.2		(0.7)	(2.4)
Income before income taxes		1.2		(0.7)
Provision for income taxes		0.4		(0.2)
Net income including noncontrolling interest		0.8		(0.5)
Net income attributable to Ecolab		0.8		(0.5)
Adjustment | Pension Standard Adoption
Cost of sales (including special charges)		1.3		3.7
Selling, general and administrative expenses		15.6		46.8
Operating income		(16.9)		(50.5)	$ (67.3)
Other (income)/expense		(16.9)		(50.5)
Product and equipment
Net sales	$ 3,090.3	2,931.9	$ 8,985.7	8,376.9
Cost of sales (including special charges)	1,811.4	1,676.9	5,259.9	4,834.2
Product and equipment | Adjustment | Revenue Standard Adoption
Net sales		2,931.9		8,376.9
Cost of sales (including special charges)		1,675.8		4,831.1
Product and equipment | Adjustment | Pension Standard Adoption
Cost of sales (including special charges)		1.1		3.1
Service and lease
Net sales	656.9	632.6	1,922.0	1,810.0
Cost of sales (including special charges)	$ 388.8	387.7	$ 1,178.5	1,127.6
Service and lease | Adjustment | Revenue Standard Adoption
Net sales		632.6		1,810.0
Cost of sales (including special charges)		387.5		1,127.0
Service and lease | Adjustment | Pension Standard Adoption
Cost of sales (including special charges)		$ 0.2		$ 0.6