COMMITMENTS AND CONTINGENCIES: - Other (Details) S/ in Millions, $ in Millions	1 Months Ended				12 Months Ended
	Jun. 30, 2018 USD ($) T	May 31, 2016 MW	Jul. 31, 2014 MW	Jun. 30, 2014 MW	Dec. 31, 2018 PEN (S/) item m km T	Dec. 31, 2018 USD ($) item m km T	Dec. 31, 2017 USD ($)
Other commitments
Amount accrued for project to evaluate the next 3 to 5 years						$ 68.2	$ 37.2
Commitment for capital projects						343.1
Tia Maria | Peru
Other commitments
Project budget						1,400.0
Amount spend as of the current date						$ 333.9
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					9,000	9,000
Number of direct jobs expected to be generated | item					3,600	3,600
Number of indirect jobs expected to be generated | item					5,400	5,400
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						1,227.0
Amount committed to funding for social and infrastructure improvement projects					S/ 445.0	131.7
Development Fund Moquegua Region | Peru
Other commitments
Amount committed to funding for social and infrastructure improvement projects					S/ 700.0	$ 207.2
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
Power purchase agreements | Electroperu S.A | Peru
Other commitments
Purchase agreement, contracted power capacity (in megawatts) | MW				120
Term of power purchase agreement related to sale of power plant				20 years
Power purchase agreements | Kallpa | Peru
Other commitments
Purchase agreement, contracted power capacity (in megawatts) | MW			120
Term of power purchase agreement related to sale of power plant		10 years	10 years
Power purchase agreements | Kallpa | Peru | Maximum
Other commitments
Purchase agreement, contracted power capacity (in megawatts) | MW		80
Educational Project | Development Fund Moquegua Region | Peru
Other commitments
Amount committed to funding for social and infrastructure improvement projects					S/ 108.5	$ 32.1
Water Treatment | Development Fund Moquegua Region | Peru
Other commitments
Amount committed to funding for social and infrastructure improvement projects					48.4	14.3
Social Investment For Taxes | Development Fund Moquegua Region | Peru
Other commitments
Amount committed to funding for social and infrastructure improvement projects					S/ 202.0	$ 59.8
Number of infrastructure projects agreed for construction | item					6	6