FAIR VALUE MEASUREMENT: (Details) - USD ($) $ in Millions	Mar. 31, 2020	Dec. 31, 2019
Liabilities:
Long-term debt, Carrying Value	$ 6,941.8	$ 6,940.8
Long-term debt, Fair Value	7,461.4	8,451.4
Short-term investment:
Trading securities	40.1	80.0
Available-for-sale debt securities:
Available-for-sale Securities, Current, Total	0.6	0.7
Quoted prices in active markets for identical assets (Level 1)
Liabilities:
Long-term debt, Carrying Value	6,191.4	6,190.6
Long-term debt, Fair Value	6,712.2	7,675.3
Significant other observable inputs (Level 2)
Liabilities:
Long-term debt, Carrying Value	750.4	750.2
Long-term debt, Fair Value	749.2	776.1
Fair value measurements recurring \| Fair value as of the end of the period
Short-term investment:
Trading securities	40.1	80.0
Derivative:
Total assets, fair value	353.0	534.0
Fair value measurements recurring \| Fair value as of the end of the period \| Copper
Derivative:
Provisionally priced sales	187.2	326.5
Fair value measurements recurring \| Fair value as of the end of the period \| Molybdenum
Derivative:
Provisionally priced sales	125.1	126.8
Fair value measurements recurring \| Asset backed securities \| Fair value as of the end of the period
Available-for-sale debt securities:
Available-for-sale debt securities	0.3	0.4
Fair value measurements recurring \| Mortgage backed securities \| Fair value as of the end of the period
Available-for-sale debt securities:
Available-for-sale debt securities	0.3	0.3
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1)
Short-term investment:
Trading securities	40.1	80.0
Derivative:
Total assets, fair value	352.4	533.3
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1) \| Copper
Derivative:
Provisionally priced sales	187.2	326.5
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1) \| Molybdenum
Derivative:
Provisionally priced sales	125.1	126.8
Fair value measurements recurring \| Significant other observable inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale Securities, Current, Total	0.6	0.7
Fair value measurements recurring \| Significant other observable inputs (Level 2) \| Asset backed securities
Available-for-sale debt securities:
Available-for-sale debt securities	0.3	0.4
Fair value measurements recurring \| Significant other observable inputs (Level 2) \| Mortgage backed securities
Available-for-sale debt securities:
Available-for-sale debt securities	$ 0.3	$ 0.3

Fair value portion of probable future economic benefits obtained or controlled by an entity as a result of past transactions or events.

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