RESTRUCTURING CHARGES (Details) $ in Millions	12 Months Ended			13 Months Ended	57 Months Ended	73 Months Ended
	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Jan. 18, 2021 USD ($)	Dec. 11, 2019 T	Dec. 31, 2017 USD ($)	Mar. 21, 2016 T
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 9.0	$ 76.5	$ 21.9			$ 267.5
Restructuring and Related Cost, Incurred Cost						120.5
Restructuring Reserve, Settled without Cash						143.5
Accrued Restructuring Costs	3.5	3.1	8.2	$ 3.5	$ 3.5	3.5			$ 5.1
Facility exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	5.4	14.6	12.0			59.8
Accrued Restructuring Costs	0.0	0.0	0.7	0.0	0.0	0.0			0.0
Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1.4	0.9	5.6			48.7
Accrued Restructuring Costs	1.7	3.1	6.0	1.7	1.7	1.7			$ 3.3
Freeport | Chlor Alkali Products and Vinyls
Restructuring Cost and Reserve [Line Items]
Freeport Chlor Alkali Capacity Reduction | T								230,000
Restructuring charges	3.8	58.9		62.7
Additional restructuring and related expected cost	45.0			45.0	45.0	45.0
Freeport | Chlor Alkali Products and Vinyls | Facility exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges				1.7
Freeport | Chlor Alkali Products and Vinyls | Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges				0.0
Geelong | Winchester
Restructuring Cost and Reserve [Line Items]
Restructuring charges		0.4	4.1
Other Winchester | Winchester
Restructuring Cost and Reserve [Line Items]
Restructuring charges		1.4
Capacity Reductions | Chlor Alkali Products and Vinyls
Restructuring Cost and Reserve [Line Items]
Restructuring charges	5.2	$ 15.8	15.7		180.7
Additional restructuring and related expected cost	$ 2.0			$ 2.0	2.0	2.0
Total Product Segment Production Capacity Decrease | T										433,000
Henderson Product Segment Production Capacity Decrease | T										153,000
Niagara Product Segment Production Capacity | T										300,000
Reduced Niagara Segment Production Capacity | T										240,000
Freeport Product Segment Production Capacity Decrease | T										220,000
Capacity Reductions | Chlor Alkali Products and Vinyls | Facility exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges					52.0
Capacity Reductions | Chlor Alkali Products and Vinyls | Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges					$ 42.2
Becancour | Chlor Alkali Products and Vinyls
Restructuring Cost and Reserve [Line Items]
Restructuring charges			$ 2.1			18.2
Becancour | Chlor Alkali Products and Vinyls | Facility exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges						5.9
Becancour | Chlor Alkali Products and Vinyls | Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges						$ 6.1
Tric/AnHCL | Chlor Alkali Products and Vinyls | Subsequent Event | Facility exit costs
Restructuring Cost and Reserve [Line Items]
Additional restructuring and related expected cost							$ 21.0
Tric/AnHCL | Chlor Alkali Products and Vinyls | Subsequent Event | Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Additional restructuring and related expected cost							2.0
Tric/AnHCL | Chlor Alkali Products and Vinyls | Subsequent Event | 2023
Restructuring Cost and Reserve [Line Items]
Additional restructuring and related expected cost							23.0
Tric/AnHCL | Chlor Alkali Products and Vinyls | Subsequent Event | 2021
Restructuring Cost and Reserve [Line Items]
Additional restructuring and related expected cost							$ 2.0