REVENUE: - Revenue by Segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2020	Mar. 31, 2019	Sep. 30, 2018
Revenue Recognition
Net sales	$ 1,719.7	$ 1,753.4	$ 1,753.4
Corporate, other and eliminations
Revenue Recognition
Net sales	(26.8)		(19.9)
Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	953.7		1,072.1
Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	134.5		121.1
Peruvian Operations | Operating segment
Revenue Recognition
Net sales	658.3		580.1
Copper
Revenue Recognition
Net sales	1,365.8		1,415.6
Copper | Corporate, other and eliminations
Revenue Recognition
Net sales	(12.9)		(12.9)
Copper | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	799.7		895.1
Copper | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	18.0		11.0
Copper | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	561.0		522.4
Molybdenum
Revenue Recognition
Net sales	126.1		118.3
Molybdenum | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	72.0		90.1
Molybdenum | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	54.1		28.2
Silver
Revenue Recognition
Net sales	88.1		76.8
Silver | Corporate, other and eliminations
Revenue Recognition
Net sales	(11.9)		(5.8)
Silver | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	44.6		50.4
Silver | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	35.4		17.3
Silver | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	20.0		14.9
Zinc
Revenue Recognition
Net sales	64.9		76.7
Zinc | Corporate, other and eliminations
Revenue Recognition
Net sales	(0.5)		(0.4)
Zinc | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	65.4		77.1
Other
Revenue Recognition
Net sales	74.8		66.0
Other | Corporate, other and eliminations
Revenue Recognition
Net sales	(1.5)		(0.8)
Other | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	37.4		36.5
Other | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	15.7		15.7
Other | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	$ 23.2		$ 14.6