CONCENTRATION OF RISK: (Details) $ in Millions	12 Months Ended
	Dec. 31, 2017 USD ($) item	Dec. 31, 2016	Dec. 31, 2015
Concentration of risk
Number of open-pit copper mines | item	4
Number of underground poly metallic mines | item	5
Number of smelters | item	2
Number of refineries | item	8
Cash equivalents
Concentration of risk
Total cash and short-term investment	$ 1,055.3
Percentage of total cash	100.00%
Percentage of cash in US dollars	94.90%
Cash equivalents | United States
Concentration of risk
Total cash and short-term investment	$ 655.1
Percentage of total cash	62.10%
Percentage invested in one institution of country	19.20%
Percentage invested in one institution of total cash	11.90%
Cash equivalents | Switzerland
Concentration of risk
Total cash and short-term investment	$ 319.5
Percentage of total cash	30.30%
Percentage invested in one institution of country	60.60%
Percentage invested in one institution of total cash	18.30%
Cash equivalents | Peru
Concentration of risk
Total cash and short-term investment	$ 23.5
Percentage of total cash	2.20%
Percentage invested in one institution of country	63.50%
Percentage invested in one institution of total cash	1.40%
Cash equivalents | Mexico
Concentration of risk
Total cash and short-term investment	$ 57.2
Percentage of total cash	5.40%
Percentage invested in one institution of country	61.40%
Percentage invested in one institution of total cash	3.30%
Accounts receivable trade | Five largest customers
Concentration of risk
Percentage of accounts receivable and total sales	28.40%	31.10%	32.40%
Accounts receivable trade | Largest customer
Concentration of risk
Percentage of accounts receivable and total sales	8.20%	11.90%	10.90%
Total sales | Five largest customers
Concentration of risk
Percentage of accounts receivable and total sales	25.10%	30.40%	28.00%
Total sales | Largest customer
Concentration of risk
Percentage of accounts receivable and total sales	8.80%	8.40%	7.70%