OTHER ASSETS (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020	Dec. 31, 2020
Other Assets [Abstract]
Supply contracts	$ 1,076.8	$ 1,129.7	$ 1,076.8	$ 1,129.7	$ 1,122.9
Other	75.6	47.3	75.6	47.3	68.4
Other assets	1,152.4	1,177.0	1,152.4	1,177.0	$ 1,191.3
2020 Supply Contract Payment				461.0
2020 Supply Contract Payment Accretion Expense				4.0
Payments under other long-term supply contracts			0.0	75.8
Amortization of Supply Contracts	$ 17.3	$ 11.3	$ 52.1	$ 44.6