REVENUE RECOGNITION: - Revenue by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
Revenue Recognition
Net sales	$ 1,723.7	$ 1,676.5	$ 5,402.1	$ 4,790.2
Corporate, other and eliminations
Revenue Recognition
Net sales	(17.2)	(16.0)	(55.6)	(54.1)
Mexican Open-pit
Revenue Recognition
Net sales	995.1	1,023.3	3,124.9	2,875.3
Mexican Open-pit \| Operating segment
Revenue Recognition
Net sales	995.1	1,023.3	3,124.9	2,875.3
Mexican IMMSA Unit
Revenue Recognition
Net sales	90.1	87.5	354.0	305.0
Mexican IMMSA Unit \| Operating segment
Revenue Recognition
Net sales	107.3	103.5	409.6	359.1
Peruvian Operations
Revenue Recognition
Net sales	638.5	565.7	1,923.2	1,609.9
Peruvian Operations \| Operating segment
Revenue Recognition
Net sales	638.5	565.7	1,923.2	1,609.9
Copper
Revenue Recognition
Net sales	1,391.5	1,401.5	4,353.4	3,943.4
Copper \| Corporate, other and eliminations
Revenue Recognition
Net sales	(10.0)	(9.2)	(32.2)	(28.2)
Copper \| Mexican Open-pit \| Operating segment
Revenue Recognition
Net sales	840.8	898.9	2,661.9	2,515.5
Copper \| Mexican IMMSA Unit \| Operating segment
Revenue Recognition
Net sales	9.9	9.2	37.0	28.1
Copper \| Peruvian Operations \| Operating segment
Revenue Recognition
Net sales	550.8	502.6	1,686.7	1,428.0
Molybdenum
Revenue Recognition
Net sales	137.6	91.2	378.5	251.1
Molybdenum \| Mexican Open-pit \| Operating segment
Revenue Recognition
Net sales	89.2	57.3	254.2	153.9
Molybdenum \| Peruvian Operations \| Operating segment
Revenue Recognition
Net sales	48.4	33.9	124.3	97.2
Zinc
Revenue Recognition
Net sales	63.2	60.0	252.9	215.7
Zinc \| Corporate, other and eliminations
Revenue Recognition
Net sales		0.2	(0.1)	0.2
Zinc \| Mexican IMMSA Unit \| Operating segment
Revenue Recognition
Net sales	63.2	59.8	253.0	215.5
Silver
Revenue Recognition
Net sales	71.7	70.7	226.0	220.0
Silver \| Corporate, other and eliminations
Revenue Recognition
Net sales	(6.6)	(5.8)	(21.2)	(20.7)
Silver \| Mexican Open-pit \| Operating segment
Revenue Recognition
Net sales	42.5	42.3	133.2	132.7
Silver \| Mexican IMMSA Unit \| Operating segment
Revenue Recognition
Net sales	19.0	16.5	63.4	56.4
Silver \| Peruvian Operations \| Operating segment
Revenue Recognition
Net sales	16.8	17.7	50.6	51.6
Other
Revenue Recognition
Net sales	59.7	53.1	191.3	160.0
Other \| Corporate, other and eliminations
Revenue Recognition
Net sales	(0.6)	(1.2)	(2.1)	(5.4)
Other \| Mexican Open-pit \| Operating segment
Revenue Recognition
Net sales	22.6	24.8	75.6	73.2
Other \| Mexican IMMSA Unit \| Operating segment
Revenue Recognition
Net sales	15.2	18.0	56.2	59.1
Other \| Peruvian Operations \| Operating segment
Revenue Recognition
Net sales	$ 22.5	$ 11.5	$ 61.6	$ 33.1