REVENUE: - Revenue by Geographical Areas (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
Mexico
Revenue Recognition
Net sales	436.0		418.2
Mexico | Corporate, other and eliminations
Revenue Recognition
Net sales	(26.8)		(19.9)
Mexico | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	363.6		351.3
Mexico | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	99.2		86.8
United States
Revenue Recognition
Net sales	299.3		287.1
United States | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	258.2		279.0
United States | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	0.2		1.4
United States | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	40.9		6.7
Peru
Revenue Recognition
Net sales	76.1		87.3
Peru | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales			1.6
Peru | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	3.0
Peru | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	73.1		85.7
Brazil
Revenue Recognition
Net sales	58.0		55.5
Brazil | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	3.5		7.2
Brazil | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	54.5		48.3
Chile
Revenue Recognition
Net sales	50.1		21.1
Chile | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	1.9		1.1
Chile | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	48.2		20.0
Other American countries
Revenue Recognition
Net sales	9.4		15.1
Other American countries | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	8.3		12.8
Other American countries | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	0.9		0.5
Other American countries | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	0.2		1.8
Switzerland
Revenue Recognition
Net sales	253.9		347.8
Switzerland | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	172.5		225.4
Switzerland | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	14.9		12.0
Switzerland | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	66.5		110.4
Italy
Revenue Recognition
Net sales	70.2		82.0
Italy | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales			29.2
Italy | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	3.4		5.5
Italy | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	66.8		47.3
Spain
Revenue Recognition
Net sales	46.6		45.5
Spain | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	46.6		45.5
Other European Countries
Revenue Recognition
Net sales	79.3		82.6
Other European Countries | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	20.3		22.9
Other European Countries | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	6.1		5.8
Other European Countries | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	52.9		53.9
Singapore
Revenue Recognition
Net sales	200.7		112.9
Singapore | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	61.7		58.5
Singapore | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	3.2		1.8
Singapore | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	135.8		52.6
Japan
Revenue Recognition
Net sales	108.1		127.3
Japan | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	9.4		9.8
Japan | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	98.7		117.5
Other Asian countries
Revenue Recognition
Net sales	32.0		71.0
Other Asian countries | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	11.2		35.0
Other Asian countries | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	0.1		0.1
Other Asian countries | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	$ 20.7		$ 35.9