SEGMENT INFORMATION (Details 3) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Disaggregation of Revenue [Line Items]
Sales	$ 1,654.1	$ 1,437.6	$ 1,241.2	$ 1,425.1	$ 1,387.1	$ 1,576.6	$ 1,592.9	$ 1,553.4	$ 5,758.0	$ 6,110.0	$ 6,946.1
U.S.
Disaggregation of Revenue [Line Items]
Sales									3,697.2	3,599.5	3,944.4
Europe
Disaggregation of Revenue [Line Items]
Sales									798.1	990.4	1,183.9
Other foreign
Disaggregation of Revenue [Line Items]
Sales									1,262.7	1,520.1	1,817.8
Chlor Alkali Products and Vinyls
Disaggregation of Revenue [Line Items]
Sales									2,959.9	3,420.1	3,986.7
Chlor Alkali Products and Vinyls | Caustic soda
Disaggregation of Revenue [Line Items]
Sales									1,408.3	1,737.4	2,198.6
Chlor Alkali Products and Vinyls | Chlorine, chlorine derivatives and other co-products
Disaggregation of Revenue [Line Items]
Sales									1,551.6	1,682.7	1,788.1
Chlor Alkali Products and Vinyls | U.S.
Disaggregation of Revenue [Line Items]
Sales									2,208.5	2,332.1	2,610.7
Chlor Alkali Products and Vinyls | Europe
Disaggregation of Revenue [Line Items]
Sales									104.4	134.5	181.8
Chlor Alkali Products and Vinyls | Other foreign
Disaggregation of Revenue [Line Items]
Sales									647.0	953.5	1,194.2
Epoxy
Disaggregation of Revenue [Line Items]
Sales									1,870.5	2,024.4	2,303.1
Epoxy | Aromatics and allylics
Disaggregation of Revenue [Line Items]
Sales									821.0	945.3	1,145.7
Epoxy | Epoxy resins
Disaggregation of Revenue [Line Items]
Sales									1,049.5	1,079.1	1,157.4
Epoxy | U.S.
Disaggregation of Revenue [Line Items]
Sales									622.8	664.0	742.7
Epoxy | Europe
Disaggregation of Revenue [Line Items]
Sales									684.4	844.2	991.1
Epoxy | Other foreign
Disaggregation of Revenue [Line Items]
Sales									563.3	516.2	569.3
Winchester
Disaggregation of Revenue [Line Items]
Sales									927.6	665.5	656.3
Winchester | Commercial
Disaggregation of Revenue [Line Items]
Sales									640.5	441.3	427.6
Winchester | Military and law enforcement
Disaggregation of Revenue [Line Items]
Sales									287.1	224.2	228.7
Winchester | U.S.
Disaggregation of Revenue [Line Items]
Sales									865.9	603.4	591.0
Winchester | Europe
Disaggregation of Revenue [Line Items]
Sales									9.3	11.7	11.0
Winchester | Other foreign
Disaggregation of Revenue [Line Items]
Sales									$ 52.4	$ 50.4	$ 54.3