REVENUE: - Revenue by Geographical Areas (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
Mexico
Revenue Recognition
Net sales	374.3	401.5	1,066.5	1,199.9
Mexico | Corporate, other and eliminations
Revenue Recognition
Net sales	(29.7)	(18.6)	(78.0)	(63.9)
Mexico | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	319.5	338.3	890.3	1,002.5
Mexico | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	84.5	81.8	254.2	261.3
United States
Revenue Recognition
Net sales	354.8	342.2	976.3	910.5
United States | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	309.9	280.4	866.8	817.3
United States | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	11.3	3.1	15.5	6.2
United States | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	33.6	58.7	94.0	87.0
Peru
Revenue Recognition
Net sales	99.9	75.2	218.6	255.6
Peru | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales				1.6
Peru | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	4.8		10.4
Peru | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	95.1	75.2	208.2	254.0
Brazil
Revenue Recognition
Net sales	65.6	49.7	145.9	154.9
Brazil | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	6.7	1.0	11.6	13.3
Brazil | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	58.9	48.7	134.3	141.6
Chile
Revenue Recognition
Net sales	55.4	64.5	164.2	144.9
Chile | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales		0.0	19.5	1.3
Chile | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	55.4	64.4	144.7	143.6
Other American countries
Revenue Recognition
Net sales	18.6	11.2	29.4	39.2
Other American countries | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	16.9	10.1	25.9	34.1
Other American countries | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	0.7	0.7	2.1	2.0
Other American countries | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	1.0	0.4	1.4	3.1
Switzerland
Revenue Recognition
Net sales	446.6	263.9	1,100.4	872.7
Switzerland | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	274.0	171.1	682.6	579.4
Switzerland | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	19.2	11.3	50.1	28.4
Switzerland | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	153.4	81.5	367.7	264.9
Italy
Revenue Recognition
Net sales	62.2	62.3	185.4	218.6
Italy | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales		5.7		42.7
Italy | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	2.0	2.0	6.3	10.3
Italy | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	60.2	54.6	179.1	165.6
Spain
Revenue Recognition
Net sales	62.5	46.7	127.3	139.0
Spain | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	62.5	46.7	127.3	139.0
Other European Countries
Revenue Recognition
Net sales	136.6	67.2	342.6	248.5
Other European Countries | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	42.4	24.8	118.8	70.3
Other European Countries | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	11.0	4.4	20.7	19.2
Other European Countries | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	83.2	38.0	203.1	159.0
Singapore
Revenue Recognition
Net sales	192.7	283.1	622.6	667.8
Singapore | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	55.9	68.0	233.9	198.5
Singapore | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	0.6	2.3	5.7	8.7
Singapore | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	136.2	212.8	383.0	460.6
Japan
Revenue Recognition
Net sales	144.3	127.9	392.8	387.9
Japan | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	15.2	21.3	26.9	37.3
Japan | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	129.1	106.6	365.9	350.6
Other Asian countries
Revenue Recognition
Net sales	115.6	64.1	262.2	191.5
Other Asian countries | Mexican Open-pit | Operating segment
Revenue Recognition
Net sales	85.4	15.9	160.1	85.1
Other Asian countries | Mexican IMMSA Unit | Operating segment
Revenue Recognition
Net sales	0.1	0.1	0.4	0.7
Other Asian countries | Peruvian Operations | Operating segment
Revenue Recognition
Net sales	$ 30.1	$ 48.1	$ 101.7	$ 105.7