SEGMENT AND RELATED INFORMATION: (Details) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($) segment	Jun. 30, 2024 USD ($)	Dec. 31, 2024 USD ($)
Financial information related to segments
Number of reportable segments | segment			3
Financial information relating to segments
Net sales	$ 3,051.0	$ 3,118.3	$ 6,172.9	$ 5,718.1
Cost of sales (exclusive of depreciation, amortization and depletion)	1,211.7	1,248.9	2,530.9	2,406.5
Selling, general and administrative	32.5	33.9	64.1	64.8
Depreciation, amortization and depletion	206.2	209.6	430.0	418.6
Exploration	13.7	18.6	25.4	31.2
Operating income	1,587.0	1,607.3	3,122.5	2,797.0
Interest, net	(40.8)	(56.8)	(84.1)	(109.9)
Other income (expense)	(2.3)	(19.9)	(16.0)	(0.9)
Income before income taxes	1,543.9	1,530.6	3,022.4	2,686.2
Capital investment	235.7	331.8	553.5	545.6
Property and mine development, net	9,915.0	9,871.5	9,915.0	9,871.5	$ 9,883.3
Total assets	19,554.7	17,777.7	19,554.7	17,777.7	$ 18,713.5
Operating segment
Financial information relating to segments
Net sales	3,112.3	3,170.5	6,283.1	5,811.4
Cost of sales (exclusive of depreciation, amortization and depletion)	1,285.7	1,299.0	2,662.8	2,497.9
Selling, general and administrative	29.9	31.1	58.5	60.2
Depreciation, amortization and depletion	193.4	200.7	405.4	400.9
Exploration	13.5	15.7	21.0	26.8
Operating income	1,589.8	1,623.9	3,135.3	2,825.6
Capital investment	234.4	330.0	550.6	539.4
Property and mine development, net	9,470.3	9,229.0	9,470.3	9,229.0
Total assets	15,577.4	16,114.7	15,577.4	16,114.7
Corporate, Other & Eliminations
Financial information relating to segments
Net sales	(61.3)	(52.1)	(110.2)	(93.4)
Cost of sales (exclusive of depreciation, amortization and depletion)	(74.0)	(50.1)	(132.0)	(91.5)
Selling, general and administrative	2.6	2.8	5.6	4.6
Depreciation, amortization and depletion	12.8	9.0	24.5	17.7
Exploration	0.1	2.9	4.4	4.4
Operating income	(2.8)	(16.7)	(12.8)	(28.7)
Capital investment	1.2	1.8	2.9	6.2
Property and mine development, net	444.6	642.6	444.6	642.6
Total assets	3,977.3	1,663.0	3,977.3	1,663.0
Reportable subsegments
Financial information relating to segments
Net sales	3,051.0	3,118.3	6,172.9	5,718.1
Reportable subsegments | Operating segment
Financial information relating to segments
Net sales	3,051.0	3,118.3	6,172.9	5,718.1
Intersegment sales
Financial information relating to segments
Net sales	0.0	0.0	0.0	0.0
Intersegment sales | Operating segment
Financial information relating to segments
Net sales	61.3	52.1	110.2	93.4
Intersegment sales | Corporate, Other & Eliminations
Financial information relating to segments
Net sales	(61.3)	(52.1)	(110.2)	(93.4)
Mexican Open-pit | Operating segment
Financial information relating to segments
Net sales	1,754.8	1,738.5	3,504.2	3,204.4
Cost of sales (exclusive of depreciation, amortization and depletion)	650.7	663.0	1,318.7	1,257.6
Selling, general and administrative	17.2	18.1	34.1	35.2
Depreciation, amortization and depletion	96.5	102.6	194.2	202.3
Exploration	1.5	2.4	2.7	6.0
Operating income	988.9	952.4	1,954.4	1,703.3
Capital investment	148.6	234.7	283.2	341.7
Property and mine development, net	4,899.5	4,687.2	4,899.5	4,687.2
Total assets	9,072.7	9,164.4	9,072.7	9,164.4
Mexican Open-pit | Reportable subsegments | Operating segment
Financial information relating to segments
Net sales	1,754.8	1,738.5	3,504.2	3,204.4
Mexican IMMSA Unit | Operating segment
Financial information relating to segments
Net sales	179.3	185.2	334.7	331.6
Cost of sales (exclusive of depreciation, amortization and depletion)	134.1	109.9	246.1	239.6
Selling, general and administrative	3.0	3.3	5.8	5.9
Depreciation, amortization and depletion	20.8	18.2	40.6	37.4
Exploration	3.6	2.6	6.1	4.4
Operating income	17.8	51.1	36.2	44.4
Capital investment	23.9	29.4	45.3	61.2
Property and mine development, net	780.5	775.0	780.5	775.0
Total assets	1,244.7	1,214.7	1,244.7	1,214.7
Mexican IMMSA Unit | Reportable subsegments | Operating segment
Financial information relating to segments
Net sales	118.0	133.1	224.5	238.2
Mexican IMMSA Unit | Intersegment sales | Operating segment
Financial information relating to segments
Net sales	61.3	52.1	110.2	93.4
Peruvian Operations | Operating segment
Financial information relating to segments
Net sales	1,178.2	1,246.7	2,444.2	2,275.4
Cost of sales (exclusive of depreciation, amortization and depletion)	500.9	526.1	1,098.0	1,000.7
Selling, general and administrative	9.7	9.7	18.7	19.1
Depreciation, amortization and depletion	76.1	79.8	170.7	161.2
Exploration	8.4	10.7	12.2	16.5
Operating income	583.1	620.4	1,144.6	1,077.9
Capital investment	61.9	65.9	222.1	136.4
Property and mine development, net	3,790.3	3,766.7	3,790.3	3,766.7
Total assets	5,260.0	5,735.6	5,260.0	5,735.6
Peruvian Operations | Reportable subsegments | Operating segment
Financial information relating to segments
Net sales	$ 1,178.2	$ 1,246.7	$ 2,444.2	$ 2,275.4