REVENUE RECOGNITION: - Revenue by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Revenue Recognition
Net sales	$ 1,837.2	$ 1,529.8	$ 3,678.4	$ 3,113.8
Corporate, other and eliminations
Revenue Recognition
Net sales	(18.8)	(17.6)	(38.4)	(38.1)
Mexican Open-pit
Revenue Recognition
Net sales	1,061.1	899.8	2,129.7	1,852.0
Mexican Open-pit \| Operating segment
Revenue Recognition
Net sales	1,061.1	899.8	2,129.7	1,852.0
Mexican IMMSA Unit
Revenue Recognition
Net sales	134.3	107.3	264.0	217.6
Mexican IMMSA Unit \| Operating segment
Revenue Recognition
Net sales	153.1	124.9	302.4	255.7
Peruvian Operations
Revenue Recognition
Net sales	641.8	522.7	1,284.7	1,044.2
Peruvian Operations \| Operating segment
Revenue Recognition
Net sales	641.8	522.7	1,284.7	1,044.2
Copper
Revenue Recognition
Net sales	1,488.5	1,256.1	2,961.9	2,541.9
Copper \| Corporate, other and eliminations
Revenue Recognition
Net sales	(11.0)	(8.1)	(22.2)	(19.0)
Copper \| Mexican Open-pit \| Operating segment
Revenue Recognition
Net sales	911.7	788.4	1,821.1	1,616.6
Copper \| Mexican IMMSA Unit \| Operating segment
Revenue Recognition
Net sales	15.9	8.0	27.1	18.9
Copper \| Peruvian Operations \| Operating segment
Revenue Recognition
Net sales	571.9	467.8	1,135.9	925.4
Molybdenum
Revenue Recognition
Net sales	104.5	67.1	240.8	159.9
Molybdenum \| Mexican Open-pit \| Operating segment
Revenue Recognition
Net sales	73.9	42.7	164.9	96.6
Molybdenum \| Peruvian Operations \| Operating segment
Revenue Recognition
Net sales	30.6	24.4	75.9	63.3
Zinc
Revenue Recognition
Net sales	94.8	76.7	189.7	155.7
Zinc \| Corporate, other and eliminations
Revenue Recognition
Net sales		0.3	(0.1)
Zinc \| Mexican IMMSA Unit \| Operating segment
Revenue Recognition
Net sales	94.8	76.4	189.8	155.7
Silver
Revenue Recognition
Net sales	83.3	75.5	154.4	149.3
Silver \| Corporate, other and eliminations
Revenue Recognition
Net sales	(7.1)	(7.5)	(14.6)	(14.9)
Silver \| Mexican Open-pit \| Operating segment
Revenue Recognition
Net sales	48.1	42.9	90.7	90.4
Silver \| Mexican IMMSA Unit \| Operating segment
Revenue Recognition
Net sales	23.9	20.2	44.5	39.9
Silver \| Peruvian Operations \| Operating segment
Revenue Recognition
Net sales	18.4	19.9	33.8	33.9
Other
Revenue Recognition
Net sales	66.1	54.4	131.6	107.0
Other \| Corporate, other and eliminations
Revenue Recognition
Net sales	(0.7)	(2.3)	(1.5)	(4.2)
Other \| Mexican Open-pit \| Operating segment
Revenue Recognition
Net sales	27.4	25.8	53.0	48.4
Other \| Mexican IMMSA Unit \| Operating segment
Revenue Recognition
Net sales	18.5	20.3	41.0	41.2
Other \| Peruvian Operations \| Operating segment
Revenue Recognition
Net sales	$ 20.9	$ 10.6	$ 39.1	$ 21.6