RESTRUCTURING CHARGES (Details 3) - USD ($) $ 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	Jun. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2021
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
Write-off of equipment and facility
Restructuring Cost and Reserve [Line Items]
Restructuring charges													137.0
Employee severance and related benefit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.2	10.1	0.0	0.0	0.0	0.1							19.1
Facility exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	2.9	3.4	2.3	1.0	1.7	1.5							62.3
Employee relocation costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges													1.7
Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.5	$ 0.5	$ 4.6	0.3	$ 0.0	$ 0.1							47.8
McIntosh Plan | Chlor Alkali Products and Vinyls
Restructuring Cost and Reserve [Line Items]
Restructuring charges	0.4							$ 5.2		5.2
McIntosh Plan | Chlor Alkali Products and Vinyls | Write-off of equipment and facility
Restructuring Cost and Reserve [Line Items]
Restructuring charges								0.0
McIntosh Plan | Chlor Alkali Products and Vinyls | Employee severance and related benefit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges								0.0
McIntosh Plan | Chlor Alkali Products and Vinyls | Facility exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges								0.0
McIntosh Plan | Chlor Alkali Products and Vinyls | Employee relocation costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges								0.0
McIntosh Plan | Chlor Alkali Products and Vinyls | Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges								$ 5.2
Freeport 2021 Plan | Chlor Alkali Products and Vinyls
Restructuring Cost and Reserve [Line Items]
Restructuring charges	1.6								$ 5.7	5.7
Freeport 2021 Plan | Chlor Alkali Products and Vinyls | Write-off of equipment and facility
Restructuring Cost and Reserve [Line Items]
Restructuring charges									0.0
Freeport 2021 Plan | Chlor Alkali Products and Vinyls | Employee severance and related benefit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges									0.0
Freeport 2021 Plan | Chlor Alkali Products and Vinyls | Facility exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges									5.7
Freeport 2021 Plan | Chlor Alkali Products and Vinyls | Employee relocation costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges									0.0
Freeport 2021 Plan | Chlor Alkali Products and Vinyls | Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges									$ 0.0
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
Freeport 2019 Plan | Chlor Alkali Products and Vinyls | Write-off of equipment and facility
Restructuring Cost and Reserve [Line Items]
Restructuring charges												58.9
Freeport 2019 Plan | Chlor Alkali Products and Vinyls | Employee severance and related benefit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges												2.1
Freeport 2019 Plan | Chlor Alkali Products and Vinyls | Facility exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges												3.9
Freeport 2019 Plan | Chlor Alkali Products and Vinyls | Employee relocation costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges												0.0
Freeport 2019 Plan | Chlor Alkali Products and Vinyls | Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges												$ 0.0
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
Chlor Alkali 2016 Plan | Chlor Alkali Products and Vinyls | Write-off of equipment and facility
Restructuring Cost and Reserve [Line Items]
Restructuring charges													78.1
Chlor Alkali 2016 Plan | Chlor Alkali Products and Vinyls | Employee severance and related benefit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges													6.7
Chlor Alkali 2016 Plan | Chlor Alkali Products and Vinyls | Facility exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges													52.7
Chlor Alkali 2016 Plan | Chlor Alkali Products and Vinyls | Employee relocation costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges													1.7
Chlor Alkali 2016 Plan | Chlor Alkali Products and Vinyls | Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges													$ 42.6
Productivity Plan | Corporate/other
Restructuring Cost and Reserve [Line Items]
Restructuring charges	$ 0.2						$ 10.3			$ 10.3
Productivity Plan | Corporate/other | Write-off of equipment and facility
Restructuring Cost and Reserve [Line Items]
Restructuring charges							0.0
Productivity Plan | Corporate/other | Employee severance and related benefit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges							10.3
Productivity Plan | Corporate/other | Facility exit costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges							0.0
Productivity Plan | Corporate/other | Employee relocation costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges							0.0
Productivity Plan | Corporate/other | Lease and other contract termination costs
Restructuring Cost and Reserve [Line Items]
Restructuring charges							$ 0.0