SEGMENT AND RELATED INFORMATION: - Sales (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
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)	$ 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
Copper
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									5,707.8	5,476.5	4,220.8
Molybdenum
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									509.9	353.4	268.0
Silver
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									300.2	286.3	294.3
Zinc
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									328.6	326.6	234.4
Other
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									250.2	211.7	362.3
Corporate, other and eliminations
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									(79.3)	(71.8)	(72.0)
Corporate, other and eliminations | Copper
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									(48.1)	(37.3)	(32.0)
Corporate, other and eliminations | Silver
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									(28.1)	(26.9)	(31.6)
Corporate, other and eliminations | Zinc
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									(0.1)	(0.6)
Corporate, other and eliminations | Other
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									(3.0)	(7.0)	(8.4)
Mexican Open-pit
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									4,075.9	3,972.7	3,234.3
Mexican Open-pit | Operating segment
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									4,075.9	3,972.7	3,234.3
Mexican Open-pit | Operating segment | Copper
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									3,459.1	3,480.2	2,663.1
Mexican Open-pit | Operating segment | Molybdenum
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									342.5	224.0	144.0
Mexican Open-pit | Operating segment | Silver
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									175.3	170.7	182.3
Mexican Open-pit | Operating segment | Other
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									99.0	97.8	244.9
Mexican IMMSA Unit
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									527.9	509.5	423.1
Mexican IMMSA Unit | Operating segment
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									527.9	509.5	423.1
Mexican IMMSA Unit | Operating segment | Copper
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									45.6	37.2	32.0
Mexican IMMSA Unit | Operating segment | Silver
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									83.5	71.9	82.1
Mexican IMMSA Unit | Operating segment | Zinc
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									328.7	327.2	234.4
Mexican IMMSA Unit | Operating segment | Other
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									70.1	73.2	74.6
Peruvian Operations
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									2,572.2	2,244.1	1,794.4
Peruvian Operations | Operating segment
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									2,572.2	2,244.1	1,794.4
Peruvian Operations | Operating segment | Copper
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									2,251.2	1,996.4	1,557.7
Peruvian Operations | Operating segment | Molybdenum
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									167.4	129.4	124.0
Peruvian Operations | Operating segment | Silver
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									69.5	70.6	61.5
Peruvian Operations | Operating segment | Other
SALES VALUE PER SEGMENT:
Net sales (including sales to related parties, see note 16)									$ 84.1	$ 47.7	$ 51.2