SEGMENT AND RELATED INFORMATION: (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020 USD ($)	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) segment	Jun. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Financial information related to segments
Number of reportable segments \| segment			3
Financial information relating to segments
Net sales	$ 1,785.4	$ 1,818.0	$ 3,505.1	$ 3,571.4
Cost of sales (exclusive of depreciation, amortization and depletion)	976.7	867.3	1,932.4	1,711.4
Selling, general and administrative	31.4	30.9	60.6	59.4
Depreciation, amortization and depletion	193.8	198.8	386.8	380.4
Exploration	6.3	7.4	14.9	12.9
Operating income	577.2	713.6	1,110.4	1,407.3
Interest, net	(87.8)	(79.6)	(176.6)	(153.7)
Other income (expense)	(1.0)	23.0	(8.4)	28.3
Income taxes	(224.4)	(251.1)	(446.2)	(489.0)
Equity earnings of affiliate	(3.1)	(1.5)	(2.1)	0.7
Non-controlling interest	(1.4)	(2.0)	(2.8)	(3.0)
Net income attributable to SCC	259.5	402.4	474.3	790.6
Capital investment	113.3	180.4	214.3	353.5
Property and mine development, net	9,197.0	9,414.6	9,197.0	9,414.6	$ 9,371.0
Total assets	15,805.0	15,344.6	15,805.0	15,344.6	$ 16,407.4
Intersegment sales
Financial information relating to segments
Net sales	(21.5)	(25.4)	(48.3)	(45.3)
Corporate, other and eliminations
Financial information relating to segments
Net sales	(21.5)	(25.4)	(48.3)	(45.3)
Cost of sales (exclusive of depreciation, amortization and depletion)	(27.5)	(29.9)	(60.0)	(53.4)
Selling, general and administrative	2.1	3.8	3.1	4.6
Depreciation, amortization and depletion	9.6	13.1	19.3	21.9
Exploration	0.8	0.1	4.2	0.3
Operating income	(6.5)	(12.5)	(14.9)	(18.7)
Capital investment	2.6	1.0	4.9	2.1
Property and mine development, net	514.3	364.5	514.3	364.5
Total assets	3,081.3	1,534.3	3,081.3	1,534.3
Reportable subsegments
Financial information relating to segments
Net sales	1,785.4	1,818.0	3,505.1	3,571.4
Mexican Open-pit \| Operating segment
Financial information relating to segments
Net sales	1,016.7	954.6	1,970.4	2,026.7
Cost of sales (exclusive of depreciation, amortization and depletion)	526.4	385.2	991.4	804.8
Selling, general and administrative	19.2	15.7	35.9	32.0
Depreciation, amortization and depletion	93.6	87.9	185.0	172.9
Exploration	0.8	0.5	1.5	0.9
Operating income	376.7	465.3	756.6	1,016.1
Capital investment	65.5	62.6	103.7	121.2
Property and mine development, net	4,400.1	4,710.9	4,400.1	4,710.9
Total assets	7,150.2	8,014.9	7,150.2	8,014.9
Mexican Open-pit \| Reportable subsegments \| Operating segment
Financial information relating to segments
Net sales	1,016.7	954.6	1,970.4	2,026.7
Mexican IMMSA Unit \| Operating segment
Financial information relating to segments
Net sales	101.6	122.3	236.1	243.4
Cost of sales (exclusive of depreciation, amortization and depletion)	81.9	104.5	194.1	204.5
Selling, general and administrative	1.8	2.2	4.2	4.0
Depreciation, amortization and depletion	10.3	13.3	20.9	26.2
Exploration	2.2	2.6	5.1	4.6
Operating income	5.4	(0.3)	11.8	4.1
Capital investment	14.4	22.5	39.7	47.5
Property and mine development, net	506.7	468.9	506.7	468.9
Total assets	824.1	842.1	824.1	842.1
Mexican IMMSA Unit \| Intersegment sales
Financial information relating to segments
Net sales	21.5	25.4	48.3	45.3
Mexican IMMSA Unit \| Reportable subsegments \| Operating segment
Financial information relating to segments
Net sales	80.1	96.9	187.8	198.1
Peruvian Operations \| Operating segment
Financial information relating to segments
Net sales	688.6	766.5	1,346.9	1,346.6
Cost of sales (exclusive of depreciation, amortization and depletion)	395.9	407.5	806.9	755.5
Selling, general and administrative	8.3	9.2	17.4	18.8
Depreciation, amortization and depletion	80.3	84.5	161.6	159.4
Exploration	2.5	4.2	4.1	7.1
Operating income	201.6	261.1	356.9	405.8
Capital investment	30.8	94.3	66.0	182.7
Property and mine development, net	3,775.9	3,870.3	3,775.9	3,870.3
Total assets	4,749.4	4,953.3	4,749.4	4,953.3
Peruvian Operations \| Reportable subsegments \| Operating segment
Financial information relating to segments
Net sales	$ 688.6	$ 766.5	$ 1,346.9	$ 1,346.6

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