FINANCIAL INSTRUMENTS: (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Liabilities:
Long-term debt, Carrying Value	$ 5,958.5	$ 5,957.1
Long-term debt, Fair Value	6,538.5	7,295.0
Short-term investment:
Trading securities	49.9	49.5
Available-for-sale debt securities:
Available-for-sale Securities, Current, Total	0.8	1.0
Quoted prices in active markets for identical assets (Level 1)
Liabilities:
Long-term debt, Carrying Value	5,209.5	5,208.4
Long-term debt, Fair Value	5,763.2	6,488.9
Significant other observable inputs (Level 2)
Liabilities:
Long-term debt, Carrying Value	749.0	748.7
Long-term debt, Fair Value	775.3	806.1
Fair value measurements recurring \| Fair value as of the end of the period
Short-term investment:
Trading securities	49.9	49.5
Derivative:
Total assets, fair value	248.9	337.9
Fair value measurements recurring \| Fair value as of the end of the period \| Copper
Derivative:
Provisionally priced sales	92.6	184.6
Fair value measurements recurring \| Fair value as of the end of the period \| Molybdenum
Derivative:
Provisionally priced sales	105.6	102.8
Fair value measurements recurring \| Corporate bonds \| Fair value as of the end of the period
Available-for-sale debt securities:
Available-for-sale debt securities	0.1	0.1
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	0.4	0.5
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	0.4
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1)
Short-term investment:
Trading securities	49.9	49.5
Derivative:
Total assets, fair value	248.1	336.9
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1) \| Copper
Derivative:
Provisionally priced sales	92.6	184.6
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1) \| Molybdenum
Derivative:
Provisionally priced sales	105.6	102.8
Fair value measurements recurring \| Significant other observable inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale Securities, Current, Total	0.8	1.0
Fair value measurements recurring \| Significant other observable inputs (Level 2) \| Corporate bonds
Available-for-sale debt securities:
Available-for-sale debt securities	0.1	0.1
Fair value measurements recurring \| Significant other observable inputs (Level 2) \| Asset backed securities
Available-for-sale debt securities:
Available-for-sale debt securities	0.4	0.5
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	$ 0.4

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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