OTHER ASSETS (Details) - USD ($) $ in Millions	3 Months Ended				12 Months Ended
	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2018
Other Assets [Abstract]
Supply contracts	$ 1,090.1		$ 1,127.7			$ 1,099.5
Investments in non-consolidated affiliates	0.0		29.0			8.8
Other	41.3		40.3			42.1
Other assets	1,131.4		1,197.0			$ 1,150.4
Schedule of Equity Method Investments [Line Items]
Bay Gas sale proceeds	20.0
Bay Gas realized gain on sale	11.2		0.0
2017 Supply Contract Payment				$ 209.4
Long-term Purchase Commitment [Line Items]
2020 Supply Contract Payment					$ 389.2
2020 Supply Contract Payment Total Accretion					$ 51.8
2020 Supply Contract Payment Accretion Expense	4.0		3.9
Amortization of Supply Contracts	9.4		$ 9.4
Minimum
Long-term Purchase Commitment [Line Items]
2020 Supply Contract Payment	440.0
Maximum
Long-term Purchase Commitment [Line Items]
2020 Supply Contract Payment	$ 465.0
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 non-cash impairment charge		$ 21.5
Bay Gas Sempra ownership percentage	90.90%