FAIR VALUE MEASUREMENT: (Details) - USD ($) $ in Millions	Jun. 30, 2021	Dec. 31, 2020
Liabilities:
Long-term debt, Carrying Value	$ 6,545.9	$ 6,544.2
Long-term debt, Fair Value	8,725.7	9,077.8
Short-term investment:
Trading securities	545.3	410.2
Available-for-sale debt securities:
Available-for-sale Securities, Current, Total	0.5	0.6
Quoted prices in active markets for identical assets (Level 1)
Liabilities:
Long-term debt, Carrying Value	6,195.1	6,193.6
Long-term debt, Fair Value	8,345.9	8,692.1
Significant other observable inputs (Level 2)
Liabilities:
Long-term debt, Carrying Value	350.8	350.6
Long-term debt, Fair Value	379.8	385.7
Fair value measurements recurring \| Fair value as of the end of the period
Short-term investment:
Trading securities	545.3	410.2
Derivative:
Total assets, fair value	1,605.3	1,031.9
Fair value measurements recurring \| Fair value as of the end of the period \| Copper
Derivative:
Provisionally priced sales	837.7	491.9
Fair value measurements recurring \| Fair value as of the end of the period \| Molybdenum
Derivative:
Provisionally priced sales	221.8	129.2
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	0.3
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	0.2	0.3
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1)
Short-term investment:
Trading securities	545.3	410.2
Derivative:
Total assets, fair value	1,604.8	1,031.3
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1) \| Copper
Derivative:
Provisionally priced sales	837.7	491.9
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1) \| Molybdenum
Derivative:
Provisionally priced sales	221.8	129.2
Fair value measurements recurring \| Significant other observable inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale Securities, Current, Total	0.5	0.6
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	0.3
Fair value measurements recurring \| Significant other observable inputs (Level 2) \| Mortgage backed securities
Available-for-sale debt securities:
Available-for-sale debt securities	$ 0.2	$ 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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