SEGMENT AND RELATED INFORMATION: - Geographical (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Schedule of revenue by geographical location
Net sales	$ 1,694.7	$ 1,723.7	$ 1,837.2	$ 1,841.1	$ 1,864.3	$ 1,676.5	$ 1,529.8	$ 1,583.9	$ 7,096.7	$ 6,654.5	$ 5,379.8
Mexico
Schedule of revenue by geographical location
Net sales									1,649.3	1,523.9	1,409.7
United States
Schedule of revenue by geographical location
Net sales									1,258.6	1,190.3	1,050.0
Peru
Schedule of revenue by geographical location
Net sales									390.4	374.2	294.4
Brazil
Schedule of revenue by geographical location
Net sales									271.9	240.1	196.1
Chile
Schedule of revenue by geographical location
Net sales									136.3	103.4	92.4
Other American countries
Schedule of revenue by geographical location
Net sales									65.8	91.5	74.3
Switzerland
Schedule of revenue by geographical location
Net sales									653.4	602.3	437.3
Italy
Schedule of revenue by geographical location
Net sales									347.3	332.4	304.5
Spain
Schedule of revenue by geographical location
Net sales									169.4	142.8	71.2
Other European Countries
Schedule of revenue by geographical location
Net sales									379.6	337.2	201.8
Singapore
Schedule of revenue by geographical location
Net sales									1,069.2	1,051.3	652.8
Japan
Schedule of revenue by geographical location
Net sales									483.4	479.6	409.8
Other Asian countries
Schedule of revenue by geographical location
Net sales									222.1	185.5	185.5
Corporate, other and eliminations
Schedule of revenue by geographical location
Net sales									(79.3)	(71.8)	(72.0)
Corporate, other and eliminations | Mexico
Schedule of revenue by geographical location
Net sales									(79.3)	(71.8)	(72.0)
Mexican Open-pit
Schedule of revenue by geographical location
Net sales									4,075.9	3,972.7	3,234.3
Mexican Open-pit | Mexico
Schedule of revenue by geographical location
Net sales									1,343.0	1,240.0	1,184.8
Mexican Open-pit | United States
Schedule of revenue by geographical location
Net sales									1,008.0	1,001.2	933.6
Mexican Open-pit | Other American countries
Schedule of revenue by geographical location
Net sales									60.6	68.1	52.9
Mexican Open-pit | Switzerland
Schedule of revenue by geographical location
Net sales									453.3	473.1	245.9
Mexican Open-pit | Italy
Schedule of revenue by geographical location
Net sales									20.6	27.7	20.9
Mexican Open-pit | Spain
Schedule of revenue by geographical location
Net sales									169.4	142.8	71.2
Mexican Open-pit | Other European Countries
Schedule of revenue by geographical location
Net sales									234.8	230.3	127.0
Mexican Open-pit | Singapore
Schedule of revenue by geographical location
Net sales									528.8	547.9	471.9
Mexican Open-pit | Japan
Schedule of revenue by geographical location
Net sales									71.8	93.0	49.2
Mexican Open-pit | Other Asian countries
Schedule of revenue by geographical location
Net sales									185.6	148.6	76.9
Mexican IMMSA Unit
Schedule of revenue by geographical location
Net sales									527.9	509.5	423.1
Mexican IMMSA Unit | Mexico
Schedule of revenue by geographical location
Net sales									385.6	349.1	296.9
Mexican IMMSA Unit | United States
Schedule of revenue by geographical location
Net sales									6.6	41.7	49.5
Mexican IMMSA Unit | Peru
Schedule of revenue by geographical location
Net sales										0.8
Mexican IMMSA Unit | Brazil
Schedule of revenue by geographical location
Net sales									43.8	48.7	26.1
Mexican IMMSA Unit | Other American countries
Schedule of revenue by geographical location
Net sales									3.8	4.1	2.6
Mexican IMMSA Unit | Switzerland
Schedule of revenue by geographical location
Net sales									43.4	14.3	9.4
Mexican IMMSA Unit | Italy
Schedule of revenue by geographical location
Net sales									21.7	18.6	17.2
Mexican IMMSA Unit | Other European Countries
Schedule of revenue by geographical location
Net sales									19.9	30.3	20.8
Mexican IMMSA Unit | Singapore
Schedule of revenue by geographical location
Net sales									2.2	1.4
Mexican IMMSA Unit | Other Asian countries
Schedule of revenue by geographical location
Net sales									0.9	0.5	0.6
Peruvian Operations
Schedule of revenue by geographical location
Net sales									2,572.2	2,244.1	1,794.4
Peruvian Operations | Mexico
Schedule of revenue by geographical location
Net sales										6.6
Peruvian Operations | United States
Schedule of revenue by geographical location
Net sales									244.0	147.4	66.9
Peruvian Operations | Peru
Schedule of revenue by geographical location
Net sales									390.4	373.4	294.4
Peruvian Operations | Brazil
Schedule of revenue by geographical location
Net sales									228.1	191.4	170.0
Peruvian Operations | Chile
Schedule of revenue by geographical location
Net sales									136.3	103.4	92.4
Peruvian Operations | Other American countries
Schedule of revenue by geographical location
Net sales									1.4	19.3	18.8
Peruvian Operations | Switzerland
Schedule of revenue by geographical location
Net sales									156.7	114.9	182.0
Peruvian Operations | Italy
Schedule of revenue by geographical location
Net sales									305.0	286.1	266.4
Peruvian Operations | Other European Countries
Schedule of revenue by geographical location
Net sales									124.9	76.6	54.0
Peruvian Operations | Singapore
Schedule of revenue by geographical location
Net sales									538.2	502.0	180.9
Peruvian Operations | Japan
Schedule of revenue by geographical location
Net sales									411.6	386.6	360.6
Peruvian Operations | Other Asian countries
Schedule of revenue by geographical location
Net sales									$ 35.6	$ 36.4	$ 108.0