OTHER ASSETS (Details) - USD ($) $ in Millions				12 Months Ended
	Feb. 07, 2019	Jan. 01, 2019	Oct. 05, 2015	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Mar. 31, 2019
Other Assets [Abstract]
Supply contracts				$ 1,112.6	$ 1,099.5
Investments in non-consolidated affiliates				0.0	8.8
Other				56.5	42.1
Other assets				1,169.1	1,150.4
Schedule of Equity Method Investments [Line Items]
Bay Gas gain on disposition				11.2	0.0	$ 0.0
Earnings (losses) of non-consolidated affiliates				0.0	(19.7)	1.8
Business Combination, Separately Recognized Transactions [Line Items]
2017 Supply Contract Payment						209.4
Long-term Purchase Commitment [Line Items]
2020 Supply Contract Payment						389.2
2020 Supply Contract Payment Adjustment				50.7
2020 Supply Contract Payment Total Accretion				52.7
2020 Supply Contract Accretion Expense				$ 17.0	16.0	3.9
Supply Contract Weighted-average Useful Life				20 years
Supply Contract Amortization				$ 37.6	37.6	28.2
Supply Contract Amortization Expense Next Twelve Months				38.0
Supply Contract Amortization Expense, Year 2				60.0
Supply Contract Amortization Expense, Year 3				60.0
Supply Contract Amortization Expense, Year 4				60.0
Supply Contract Amortization Expense, Year 5				60.0
Bay Gas
Schedule of Equity Method Investments [Line Items]
Bay Gas Olin ownership percentage							9.10%
Bay Gas sale proceeds		$ 20.0
Bay Gas gain on disposition	$ 11.2
Bay Gas non-cash impairment charge					21.5
Bay Gas Sempra ownership percentage							90.90%
Maximum
Long-term Purchase Commitment [Line Items]
2020 Supply Contract Payment				493.0
Chlor Alkali Products and Vinyls
Other Assets [Abstract]
Investments in non-consolidated affiliates				0.0	8.8
Chlor Alkali Products and Vinyls | Bay Gas
Schedule of Equity Method Investments [Line Items]
Earnings (losses) of non-consolidated affiliates				$ 0.0	$ (19.7)	$ 1.8
Blue Cube Business
Business Combination, Separately Recognized Transactions [Line Items]
Up-front payments under the ethylene agreements			$ 433.5
Fair value of up-front ethylene payment			$ 416.1