OPERATING SEGMENTS (Information by Product) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Segment Reporting Information [Line Items]
Sales	$ 500.3	$ 341.3	$ 245.2	$ 403.7	$ 486.5	$ 322.5	$ 246.7	$ 437.9	$ 1,490.5	$ 1,493.6	$ 1,364.4
Logistics costs			$ 2.8						2.8
Severance and other costs, gross									2.3	5.1
Restructuring charges											4.3
Reportable segments | Salt
Segment Reporting Information [Line Items]
Sales									889.5	858.1	769.2
Reportable segments | Plant Nutrition North America
Segment Reporting Information [Line Items]
Sales									206.2	233.2	210.0
Reportable segments | Plant Nutrition South America
Segment Reporting Information [Line Items]
Sales									385.1	391.8	375.0
Corporate & other
Segment Reporting Information [Line Items]
Sales									9.7	10.5	10.2
Highway Deicing Salt
Segment Reporting Information [Line Items]
Sales									545.5	532.0	455.1
Highway Deicing Salt | Reportable segments | Salt
Segment Reporting Information [Line Items]
Sales									545.5	532.0	455.1
Highway Deicing Salt | Reportable segments | Plant Nutrition North America
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
Highway Deicing Salt | Reportable segments | Plant Nutrition South America
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
Highway Deicing Salt | Corporate & other
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
Consumer & Industrial Salt
Segment Reporting Information [Line Items]
Sales									344.0	326.1	314.1
Consumer & Industrial Salt | Reportable segments | Salt
Segment Reporting Information [Line Items]
Sales									344.0	326.1	314.1
Consumer & Industrial Salt | Reportable segments | Plant Nutrition North America
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
Consumer & Industrial Salt | Reportable segments | Plant Nutrition South America
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
Consumer & Industrial Salt | Corporate & other
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
SOP and Specialty Plant Nutrients
Segment Reporting Information [Line Items]
Sales									511.2	539.0	490.1
SOP and Specialty Plant Nutrients | Reportable segments | Salt
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
SOP and Specialty Plant Nutrients | Reportable segments | Plant Nutrition North America
Segment Reporting Information [Line Items]
Sales									212.6	238.8	216.5
SOP and Specialty Plant Nutrients | Reportable segments | Plant Nutrition South America
Segment Reporting Information [Line Items]
Sales									298.6	300.2	273.6
SOP and Specialty Plant Nutrients | Corporate & other
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
Industrial Chemicals
Segment Reporting Information [Line Items]
Sales									89.2	95.0	101.4
Industrial Chemicals | Reportable segments | Salt
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
Industrial Chemicals | Reportable segments | Plant Nutrition North America
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
Industrial Chemicals | Reportable segments | Plant Nutrition South America
Segment Reporting Information [Line Items]
Sales									89.2	95.0	101.4
Industrial Chemicals | Corporate & other
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
Eliminations & Other
Segment Reporting Information [Line Items]
Sales									0.6	1.5	3.7
Eliminations & Other | Reportable segments | Salt
Segment Reporting Information [Line Items]
Sales									0.0	0.0	0.0
Eliminations & Other | Reportable segments | Plant Nutrition North America
Segment Reporting Information [Line Items]
Sales									(6.4)	(5.6)	(6.5)
Eliminations & Other | Reportable segments | Plant Nutrition South America
Segment Reporting Information [Line Items]
Sales									(2.7)	(3.4)	0.0
Eliminations & Other | Corporate & other
Segment Reporting Information [Line Items]
Sales									$ 9.7	$ 10.5	$ 10.2