COMMITMENTS AND CONTINGENCIES: - Others (Details) S/ in Millions, $ in Millions	1 Months Ended				9 Months Ended
	Jun. 30, 2018 USD ($) T	May 31, 2016 MW	Jul. 31, 2014 MW	Jun. 30, 2014 MW	Sep. 30, 2018 PEN (S/) item m km T	Sep. 30, 2018 USD ($) item m km T	Dec. 31, 2017 USD ($)
Other commitments:
Commitment for capital projects						$ 1,382.7
Amount accrued for project to evaluate the next 3 to 5 years						58.4	$ 37.2
Tia Maria | Peru
Other commitments:
Project budget						1,400.0
Amount spend as of the current date						$ 355.0
Annual production ( in tons) | T					120,000	120,000
Distance of sea water transportation | km					25	25
Height above sea level of the desalinization plant | m					1,000	1,000
Additional investment in desalinization plant						$ 95.0
Number of jobs expected to be generated | item					3,600	3,600
Number of workers expected to be directly employed | item					600	600
Number of workers expected to be indirectly employed | item					4,200	4,200
Term life of environmental project (in years)					20 years	20 years
Toquepala Concentrator Expansion | Peru
Other commitments:
Project budget						$ 1,300.0
Amount spend as of the current date						$ 1,192.4
Percentage of Work Completed (in percentage)					98.00%	98.00%
Amount committed to funding for social and infrastructure improvement projects					S/ 445.0	$ 132.0
Development Fund Moquegua Region | Peru
Other commitments:
Amount committed to funding for social and infrastructure improvement projects					S/ 700.0	$ 209.0
Copper | Michiquillay | Peru
Other commitments:
Annual production ( in tons) | T	225,000
Contingent contractual obligation	$ 400.0
Estimated mineralized material (in tons) | T	1,150,000,000
Copper grade percentage	0.63%
Initial mine life	25 years
Contractual obligation paid on project	$ 12.5
Amount accrued for project to evaluate the next 3 to 5 years	12.5
Remaining amount to pay if project is developed	$ 375.0
Copper | Toquepala Concentrator Expansion | Peru
Other commitments:
Estimated increase in annual production (in tons) | T					100,000	100,000
Molybdenum | Toquepala Concentrator Expansion | Peru
Other commitments:
Estimated increase in annual production (in tons) | T					3,100	3,100
Educational Project | Development Fund Moquegua Region | Peru
Other commitments:
Amount committed to funding for social and infrastructure improvement projects					S/ 108.5	$ 32.0
Water Treatment | Development Fund Moquegua Region | Peru
Other commitments:
Amount committed to funding for social and infrastructure improvement projects					48.4	14.0
Social Investment For Taxes | Development Fund Moquegua Region | Peru
Other commitments:
Amount committed to funding for social and infrastructure improvement projects					S/ 143.0	$ 43.0
Electroperu S.A | Power purchase agreements | Peru
Other commitments:
Term of power purchase agreement related to sale of power plant				20 years
Purchase agreement, contracted power capacity (in megawatts) | MW				120
Kallpa | Power purchase agreements | Peru
Other commitments:
Term of power purchase agreement related to sale of power plant		10 years	10 years
Purchase agreement, contracted power capacity (in megawatts) | MW			120
Kallpa | Power purchase agreements | Peru | Maximum
Other commitments:
Purchase agreement, contracted power capacity (in megawatts) | MW		80