FINANCIAL INSTRUMENTS: (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Liabilities:
Long-term debt, Carrying Value	$ 5,960.9	$ 5,960.1
Long-term debt, Fair Value	6,832.3	6,301.7
Short-term investment:
Trading securities	212.7	213.1
Available-for-sale debt securities:
Available-for-sale Securities, Current, Total	0.8	0.7
Quoted prices in active markets for identical assets (Level 1)
Liabilities:
Long-term debt, Carrying Value	5,211.3	5,210.7
Long-term debt, Fair Value	6,055.4	5,540.0
Significant other observable inputs (Level 2)
Liabilities:
Long-term debt, Carrying Value	749.6	749.4
Long-term debt, Fair Value	776.9	761.7
Fair value measurements recurring \| Fair value as of the end of the period
Short-term investment:
Trading securities	212.7	213.1
Derivative:
Total assets, fair value	646.0	595.5
Fair value measurements recurring \| Fair value as of the end of the period \| Copper
Derivative:
Provisionally priced sales	308.4	274.3
Fair value measurements recurring \| Fair value as of the end of the period \| Molybdenum
Derivative:
Provisionally priced sales	124.1	107.4
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	0.4
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	0.4	0.3
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1)
Short-term investment:
Trading securities	212.7	213.1
Derivative:
Total assets, fair value	645.2	594.8
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1) \| Copper
Derivative:
Provisionally priced sales	308.4	274.3
Fair value measurements recurring \| Quoted prices in active markets for identical assets (Level 1) \| Molybdenum
Derivative:
Provisionally priced sales	124.1	107.4
Fair value measurements recurring \| Significant other observable inputs (Level 2)
Available-for-sale debt securities:
Available-for-sale Securities, Current, Total	0.8	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	0.4	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	$ 0.4	$ 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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