OTHER ASSETS (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended		39 Months Ended
	Jan. 01, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Dec. 31, 2019	Mar. 31, 2019
Other Assets [Abstract]
Supply contracts		$ 1,137.9	$ 1,131.4	$ 1,137.9	$ 1,131.4	$ 1,137.9	$ 1,112.6
Other		41.8	43.1	41.8	43.1	41.8	56.5
Other assets		1,179.7	1,174.5	1,179.7	1,174.5	1,179.7	$ 1,169.1
Schedule of Equity Method Investments [Line Items]
Bay Gas sale proceeds					20.0
Bay Gas realized gain on sale				0.0	11.2
2020 Supply Contract Payment		461.0
2020 Supply Contract Payment Total Accretion						$ 40.9
2020 Supply Contract Payment Accretion Expense			4.2	4.0	8.2
Amortization of Supply Contracts		$ 9.4	$ 9.4	18.8	18.8
Long-term Other Supply Contract Payments				$ 75.8	0.0
Bay Gas
Schedule of Equity Method Investments [Line Items]
Bay Gas Olin ownership percentage								9.10%
Bay Gas sale proceeds	$ 20.0
Bay Gas realized gain on sale					$ 11.2