REVENUE: - Revenue by Segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue Recognition
Net sales	$ 2,129.1	$ 1,859.5	$ 5,634.2	$ 5,431.0
Corporate, other and eliminations
Revenue Recognition
Net sales	(29.7)	(18.6)	(78.0)	(63.9)
Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	1,181.7	982.4	3,152.1	3,009.1
Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	140.9	106.7	377.0	350.1
Peruvian Operations | Operating segment
Revenue Recognition
Net sales	836.2	789.0	2,183.1	2,135.7
Copper
Revenue Recognition
Net sales	1,749.9	1,477.4	4,585.4	4,341.9
Copper | Corporate, other and eliminations
Revenue Recognition
Net sales	(14.1)	(11.0)	(34.7)	(38.2)
Copper | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	1,022.1	809.6	2,687.8	2,504.9
Copper | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	21.2	11.5	50.9	34.5
Copper | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	720.7	667.3	1,881.4	1,840.7
Molybdenum
Revenue Recognition
Net sales	119.8	156.5	352.8	429.4
Molybdenum | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	58.2	91.2	188.3	274.8
Molybdenum | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	61.6	65.3	164.5	154.6
Silver
Revenue Recognition
Net sales	133.0	57.2	320.4	234.1
Silver | Corporate, other and eliminations
Revenue Recognition
Net sales	(17.5)	2.6	(38.9)	(21.3)
Silver | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	67.0		170.6	137.6
Silver | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	48.4	54.6	111.1	59.1
Silver | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	35.1		77.6	58.7
Zinc
Revenue Recognition
Net sales	62.2	88.6	175.4	209.7
Zinc | Corporate, other and eliminations
Revenue Recognition
Net sales	4.6	(9.0)	1.3	(1.6)
Zinc | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales		49.8
Zinc | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	57.6	24.7	174.1	211.3
Zinc | Peruvian Operations | Operating segment
Revenue Recognition
Net sales		23.1
Other
Revenue Recognition
Net sales	64.2	79.8	200.2	215.9
Other | Corporate, other and eliminations
Revenue Recognition
Net sales	(2.7)	(1.2)	(5.7)	(2.8)
Other | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	34.4	31.8	105.4	91.8
Other | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	13.7	15.9	40.9	45.2
Other | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	$ 18.8	$ 33.3	$ 59.6	$ 81.7