OTHER ASSETS (Details) - USD ($) $ in Millions			3 Months Ended				9 Months Ended		12 Months Ended
	Feb. 07, 2019	Jan. 01, 2019	Sep. 30, 2019	Jun. 30, 2019	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2019	Sep. 30, 2018	Dec. 31, 2017	Mar. 31, 2019	Dec. 31, 2018
Other Assets [Abstract]
Supply contracts			$ 1,122.0		$ 1,108.9		$ 1,122.0	$ 1,108.9			$ 1,099.5
Investments in non-consolidated affiliates			0.0		8.3		0.0	8.3			8.8
Other			48.1		42.4		48.1	42.4			42.1
Other assets			1,170.1		1,159.6		1,170.1	1,159.6			$ 1,150.4
Schedule of Equity Method Investments [Line Items]
Bay Gas sale proceeds							20.0	0.0
Bay Gas realized gain on sale							11.2	0.0
Bay Gas non-cash impairment charge								21.5
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 Payment Accretion Expense			4.4		4.0		12.6	11.9
Amortization of Supply Contracts			$ 9.4		$ 9.4		28.2	$ 28.2
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
Bay Gas Sempra ownership percentage			90.90%				90.90%