OTHER ASSETS (Details) - USD ($) $ in Millions			3 Months Ended			12 Months Ended
	Feb. 07, 2019	Jan. 01, 2019	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2017	Dec. 31, 2019
Other Assets [Abstract]
Supply contracts			$ 1,103.2	$ 1,090.1		$ 1,112.6
Other			102.1	41.3		56.5
Other assets			1,205.3	1,131.4		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
2017 Supply Contract Payment					$ 209.4
Long-term Purchase Commitment [Line Items]
2020 Supply Contract Payment						439.9
2020 Supply Contract Payment Total Accretion						$ 52.7
2020 Supply Contract Payment Accretion Expense			4.0	4.0
2020 Supply Contract Payment Reclassification			480.8
Amortization of Supply Contracts			9.4	$ 9.4
Maximum
Long-term Purchase Commitment [Line Items]
2020 Supply Contract Payment			$ 493.0
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