OTHER ASSETS (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017
Other Assets [Abstract]
Investments in non-consolidated affiliates	$ 8.3	$ 28.2	$ 8.3	$ 28.2	$ 28.5
Deferred debt issuance costs	2.1	2.7	2.1	2.7	2.5
Tax-related receivables	4.9	13.1	4.9	13.1	10.2
Derivative contracts	1.6	3.7	1.6	3.7	3.6
Supply contracts	1,108.9	1,146.3	1,108.9	1,146.3	1,137.1
Other	33.8	21.6	33.8	21.6	26.5
Other assets	1,159.6	1,215.6	1,159.6	1,215.6	1,208.4
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		51.8
2020 Supply Contract Payment Accretion Expense	4.0		11.9
Amortization of Supply Contracts	$ 9.4	$ 6.3	28.2	$ 18.9
Schedule of Equity Method Investments [Line Items]
BayGas non-cash impairment charge			$ 21.5
BayGas Olin ownership percentage	9.10%		9.10%
BayGas Sempra ownership percentage	90.90%		90.90%
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