RESTRUCTURING CHARGES (Details 1) $ in Millions	3 Months Ended						6 Months Ended	7 Months Ended	8 Months Ended	9 Months Ended		22 Months Ended	66 Months Ended
	Sep. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($)	Sep. 30, 2020 USD ($)	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 USD ($)	Oct. 21, 2021 USD ($) T	May 18, 2021 T	Mar. 15, 2021 T	Dec. 11, 2019 T	Mar. 21, 2016 T
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 3.6	$ 14.0	$ 6.9	$ 1.3	$ 1.7	$ 1.7				$ 24.5	$ 4.7		$ 267.9
Inception to date Amounts Utilized (cash)													115.6
Inception to date Amounts Utilized (non-cash)													137.7
Productivity Plan | Corporate/other
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.2						$ 10.3			10.3
Additional restructuring and related expected cost	0.0						0.0	$ 0.0	$ 0.0	0.0		$ 0.0	0.0
Plaquemine Plan | Chlor Alkali Products and Vinyls
Restructuring Cost and Reserve [Line Items]
Additional restructuring and related expected cost	2.0						2.0	2.0	2.0	2.0		2.0	2.0
Plaquemine Restructuring Capacity Reduction Percent															20.00%
Plaquemine Restructuring Capacity Reduction | T															225,000
McIntosh Plan | Chlor Alkali Products and Vinyls
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.4							5.2		5.2
Additional restructuring and related expected cost	2.0						2.0	2.0	2.0	2.0		2.0	2.0
McIntosh Restructuring Capacity Reduction | T																200,000
McIntosh Restructuring Capacity Reduction Percent																50.00%
McIntosh Plan | Chlor Alkali Products and Vinyls | Subsequent Event
Restructuring Cost and Reserve [Line Items]
Additional restructuring and related expected cost														$ 40.0
McIntosh Restructuring Capacity Reduction | T														200,000
McIntosh Restructuring Capacity Reduction Percent														50.00%
Freeport 2021 Plan | Chlor Alkali Products and Vinyls
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1.6								5.7	5.7
Additional restructuring and related expected cost	25.0						25.0	25.0	25.0	25.0		25.0	25.0
Freeport 2019 Plan | Chlor Alkali Products and Vinyls
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1.0			0.3						2.2	0.8	64.9
Additional restructuring and related expected cost	45.0						45.0	45.0	45.0	45.0		45.0	45.0
Freeport Chlor Alkali Capacity Reduction | T																	230,000
Chlor Alkali 2016 Plan | Chlor Alkali Products and Vinyls
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.4			$ 1.0						1.1	$ 3.9		181.8
Additional restructuring and related expected cost	$ 1.0						$ 1.0	$ 1.0	$ 1.0	$ 1.0		$ 1.0	$ 1.0
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