SEGMENT AND RELATED INFORMATION: (Details) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($) segment	Jun. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Financial information related to segments
Number of reportable segments \| segment			3
Financial information relating to segments
Net sales	$ 1,818.0	$ 1,837.2	$ 3,571.4	$ 3,678.4
Cost of sales (exclusive of depreciation, amortization and depletion)	867.3	851.7	1,711.4	1,728.2
Selling, general and administrative	30.9	26.2	59.4	50.3
Depreciation, amortization and depletion	198.8	162.6	380.4	324.6
Exploration	7.4	9.1	12.9	14.3
Operating income	713.6	787.6	1,407.3	1,561.0
Interest, net	(79.6)	(65.8)	(153.7)	(132.5)
Other income (expense)	23.0	(3.1)	28.3	(5.5)
Income taxes	(251.1)	(309.1)	(489.0)	(545.7)
Equity earnings of affiliate, net of income tax	(1.5)	1.5	0.7	5.6
Non-controlling interest	(2.0)	(1.5)	(3.0)	(2.6)
Net income attributable to SCC	402.4	409.6	790.6	880.3
Capital investment	180.4	253.9	353.5	549.6
Property and mine development, net	9,414.6	9,172.6	9,414.6	9,172.6	$ 9,403.8
Total assets	15,344.6	14,045.9	15,344.6	14,045.9	$ 14,484.8
Intersegment sales
Financial information relating to segments
Net sales	(25.4)	(18.8)	(45.3)	(38.4)
Corporate, other and eliminations
Financial information relating to segments
Net sales	(25.4)	(18.8)	(45.3)	(38.4)
Cost of sales (exclusive of depreciation, amortization and depletion)	(29.9)	(21.5)	(53.4)	(41.7)
Selling, general and administrative	3.8	1.0	4.6	1.4
Depreciation, amortization and depletion	13.1	11.7	21.9	18.4
Exploration	0.1	0.5	0.3	1.2
Operating income	(12.5)	(10.5)	(18.7)	(17.7)
Capital investment	1.0	16.6	2.1	18.7
Property and mine development, net	364.5	739.8	364.5	739.8
Total assets	1,534.3	876.0	1,534.3	876.0
Reportable subsegments
Financial information relating to segments
Net sales	1,818.0	1,837.2	3,571.4	3,678.4
Mexican Open-pit \| Operating segment
Financial information relating to segments
Net sales	954.6	1,061.1	2,026.7	2,129.7
Cost of sales (exclusive of depreciation, amortization and depletion)	385.2	404.7	804.8	816.1
Selling, general and administrative	15.7	13.7	32.0	25.6
Depreciation, amortization and depletion	87.9	81.9	172.9	174.4
Exploration	0.5	0.5	0.9	1.1
Operating income	465.3	560.3	1,016.1	1,112.5
Capital investment	62.6	79.5	121.2	144.2
Property and mine development, net	4,710.9	4,490.7	4,710.9	4,490.7
Total assets	8,014.9	7,659.1	8,014.9	7,659.1
Mexican Open-pit \| Reportable subsegments \| Operating segment
Financial information relating to segments
Net sales	954.6	1,061.1	2,026.7	2,129.7
Mexican IMMSA Unit \| Operating segment
Financial information relating to segments
Net sales	122.3	153.1	243.4	302.4
Cost of sales (exclusive of depreciation, amortization and depletion)	104.5	100.9	204.5	196.4
Selling, general and administrative	2.2	2.5	4.0	4.9
Depreciation, amortization and depletion	13.3	11.1	26.2	22.1
Exploration	2.6	1.2	4.6	2.5
Operating income	(0.3)	37.4	4.1	76.5
Capital investment	22.5	9.8	47.5	22.4
Property and mine development, net	468.9	433.2	468.9	433.2
Total assets	842.1	951.2	842.1	951.2
Mexican IMMSA Unit \| Intersegment sales
Financial information relating to segments
Net sales	25.4	18.8	45.3	38.4
Mexican IMMSA Unit \| Reportable subsegments \| Operating segment
Financial information relating to segments
Net sales	96.9	134.3	198.1	264.0
Peruvian Operations \| Operating segment
Financial information relating to segments
Net sales	766.5	641.8	1,346.6	1,284.7
Cost of sales (exclusive of depreciation, amortization and depletion)	407.5	367.6	755.5	757.4
Selling, general and administrative	9.2	9.0	18.8	18.4
Depreciation, amortization and depletion	84.5	57.9	159.4	109.7
Exploration	4.2	6.9	7.1	9.5
Operating income	261.1	200.4	405.8	389.7
Capital investment	94.3	148.0	182.7	364.3
Property and mine development, net	3,870.3	3,508.9	3,870.3	3,508.9
Total assets	4,953.3	4,559.6	4,953.3	4,559.6
Peruvian Operations \| Reportable subsegments \| Operating segment
Financial information relating to segments
Net sales	$ 766.5	$ 641.8	$ 1,346.6	$ 1,284.7

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