OTHER ASSETS (Details) - USD ($) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2018	Mar. 31, 2017	Dec. 31, 2017	Sep. 30, 2017
Other Assets [Abstract]
Investments in non-consolidated affiliates	$ 29.0	$ 27.2	$ 28.5
Deferred debt issuance costs	2.4	3.0	2.5
Tax-related receivables	5.3	16.1	10.2
Derivative contracts	4.0	8.3	3.6
Supply contracts	1,127.7	560.4	1,137.1
Other	28.6	22.2	26.5
Other assets	1,197.0	637.2	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				$ 52.0
2020 Supply Contract Payment Accretion Expense	3.9
Amortization of Supply Contracts	9.4	$ 6.3
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