FINANCIAL INSTRUMENTS: (Details) (USD $) In Millions, unless otherwise specified																													3 Months Ended
	Mar. 31, 2013	Dec. 31, 2012	Mar. 31, 2013 Fair value measurements recurring Fair value as of the end of the period	Dec. 31, 2012 Fair value measurements recurring Fair value as of the end of the period	Mar. 31, 2013 Fair value measurements recurring Fair value as of the end of the period Copper	Dec. 31, 2012 Fair value measurements recurring Fair value as of the end of the period Copper	Mar. 31, 2013 Fair value measurements recurring Fair value as of the end of the period Molybdenum	Dec. 31, 2012 Fair value measurements recurring Fair value as of the end of the period Molybdenum	Mar. 31, 2013 Fair value measurements recurring Fair value as of the end of the period Corporate bonds	Dec. 31, 2012 Fair value measurements recurring Fair value as of the end of the period Corporate bonds	Mar. 31, 2013 Fair value measurements recurring Fair value as of the end of the period Asset backed obligations	Dec. 31, 2012 Fair value measurements recurring Fair value as of the end of the period Asset backed obligations	Mar. 31, 2013 Fair value measurements recurring Fair value as of the end of the period Mortgage backed securities	Dec. 31, 2012 Fair value measurements recurring Fair value as of the end of the period Mortgage backed securities	Mar. 31, 2013 Fair value measurements recurring Quoted prices in active markets for identical assets (Level 1)	Dec. 31, 2012 Fair value measurements recurring Quoted prices in active markets for identical assets (Level 1)	Mar. 31, 2013 Fair value measurements recurring Quoted prices in active markets for identical assets (Level 1) Copper	Dec. 31, 2012 Fair value measurements recurring Quoted prices in active markets for identical assets (Level 1) Copper	Mar. 31, 2013 Fair value measurements recurring Quoted prices in active markets for identical assets (Level 1) Molybdenum	Dec. 31, 2012 Fair value measurements recurring Quoted prices in active markets for identical assets (Level 1) Molybdenum	Mar. 31, 2013 Fair value measurements recurring Significant other observable inputs (Level 2)	Dec. 31, 2012 Fair value measurements recurring Significant other observable inputs (Level 2)	Mar. 31, 2013 Fair value measurements recurring Significant other observable inputs (Level 2) Corporate bonds	Dec. 31, 2012 Fair value measurements recurring Significant other observable inputs (Level 2) Corporate bonds	Mar. 31, 2013 Fair value measurements recurring Significant other observable inputs (Level 2) Asset backed obligations	Dec. 31, 2012 Fair value measurements recurring Significant other observable inputs (Level 2) Asset backed obligations	Mar. 31, 2013 Fair value measurements recurring Significant other observable inputs (Level 2) Mortgage backed securities	Dec. 31, 2012 Fair value measurements recurring Significant other observable inputs (Level 2) Mortgage backed securities	Mar. 31, 2013 Mitsui loan
Liabilities:
Long-term debt, Carrying Value	$ 4,214.1	$ 4,213.9
Long-term debt, Fair Value	4,654.4	4,870.6
Fair values of assets and liabilities measured at fair value on a recurring basis
Fair value calculation, present value based on cash flow discount rate, weighted average cost of capital (as a percent)																													10.00%
Short term Investment:
Trading securities investment			198.0	127.8											198.0	127.8
Available-for-sale debt securities:
Available-for-sale debt securities									0.5	0.4	0.1	0.1	5.9	6.0									0.5	0.4	0.1	0.1	5.9	6.0
Derivative:
Provisionally priced sales					30.1	70.8	100.7	102.9									30.1	70.8	100.7	102.9
Total assets, fair value			$ 335.3	$ 308.0											$ 328.8	$ 301.5					$ 6.5	$ 6.5