REVENUE: - Revenue by Segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019
Revenue Recognition
Net sales	$ 1,785.4	$ 1,818.0	$ 3,505.1	$ 3,571.4
Corporate, other and eliminations
Revenue Recognition
Net sales	(21.5)	(25.4)	(48.3)	(45.3)
Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	1,016.7	954.6	1,970.4	2,026.7
Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	101.6	122.3	236.1	243.4
Peruvian Operations | Operating segment
Revenue Recognition
Net sales	688.6	766.5	1,346.9	1,346.6
Copper
Revenue Recognition
Net sales	1,469.7	1,448.9	2,835.5	2,864.5
Copper | Corporate, other and eliminations
Revenue Recognition
Net sales	(7.7)	(14.3)	(20.6)	(27.2)
Copper | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	866.0	800.2	1,665.7	1,695.3
Copper | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	11.7	12.0	29.7	23.0
Copper | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	599.7	651.0	1,160.7	1,173.4
Molybdenum
Revenue Recognition
Net sales	107.0	154.5	233.0	272.8
Molybdenum | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	58.1	93.5	130.1	183.6
Molybdenum | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	48.9	61.0	102.9	89.2
Silver
Revenue Recognition
Net sales	99.2	75.6	187.4	145.6
Silver | Corporate, other and eliminations
Revenue Recognition
Net sales	(9.5)	(3.8)	(21.4)	(12.3)
Silver | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	59.0		103.6	87.8
Silver | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	27.2	79.4	62.7	34.5
Silver | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	22.5		42.5	35.6
Zinc
Revenue Recognition
Net sales	48.4	68.8	113.2	152.3
Zinc | Corporate, other and eliminations
Revenue Recognition
Net sales	(2.8)	(6.5)	(3.3)	(4.2)
Zinc | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales		37.4
Zinc | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	51.2	17.2	116.5	156.5
Zinc | Peruvian Operations | Operating segment
Revenue Recognition
Net sales		20.7
Other
Revenue Recognition
Net sales	61.1	70.2	136.0	136.2
Other | Corporate, other and eliminations
Revenue Recognition
Net sales	(1.5)	(0.8)	(3.0)	(1.6)
Other | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	33.6	23.5	71.0	60.0
Other | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	11.5	13.7	27.2	29.4
Other | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	$ 17.5	$ 33.8	$ 40.8	$ 48.4