OTHER ASSETS (Details) - USD ($) $ in Millions			3 Months Ended	12 Months Ended
	Feb. 07, 2019	Jan. 01, 2019	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Mar. 31, 2020
Other Assets [Abstract]
Supply contracts				$ 1,122.9	$ 1,112.6
Other				68.4	56.5
Other assets				1,191.3	1,169.1
Schedule of Equity Method Investments [Line Items]
Bay Gas gain on disposition				0.0	11.2	$ 0.0
Losses of non-consolidated affiliates				0.0	0.0	(19.7)
2020 Supply Contract Payment			$ 461.0
2020 Supply Contract Payment Total Accretion				40.9
2020 Supply Contract Accretion Expense				4.0	17.0	16.0
Long-term Other Supply Contract Payments				$ 75.8	0.0	0.0
Supply Contract Weighted-average Useful Life				20 years
Supply Contract Amortization				$ 56.0	39.9	38.3
Supply Contract Amortization Expense Next Twelve Months				70.0
Supply Contract Amortization Expense, Year 2				70.0
Supply Contract Amortization Expense, Year 3				70.0
Supply Contract Amortization Expense, Year 4				70.0
Supply Contract Amortization Expense, Year 5				70.0
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%
Bay Gas | Chlor Alkali Products and Vinyls
Schedule of Equity Method Investments [Line Items]
Losses of non-consolidated affiliates				$ 0.0	$ 0.0	$ (19.7)