SEGMENT AND RELATED INFORMATION: (Details) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Sep. 30, 2019 USD ($) segment	Sep. 30, 2018 USD ($)	Dec. 31, 2018 USD ($)
Financial information related to segments
Number of reportable segments \| segment			3
Financial information relating to segments
Net sales	$ 1,859.5	$ 1,723.7	$ 5,431.0	$ 5,402.1
Cost of sales (exclusive of depreciation, amortization and depletion)	906.5	824.0	2,617.9	2,552.2
Selling, general and administrative	32.0	26.4	91.4	76.7
Depreciation, amortization and depletion	200.3	170.6	580.6	495.2
Exploration	6.9	6.0	19.9	20.3
Operating income	713.8	696.7	2,121.2	2,257.7
Interest, net	(78.7)	(64.8)	(232.5)	(197.2)
Other income (expense)	(6.3)	(7.6)	22.0	(13.1)
Income taxes	(241.0)	(257.9)	(730.0)	(803.6)
Equity earnings of affiliate	3.5	4.3	4.2	9.9
Non-controlling interest	(1.7)	(1.3)	(4.7)	(3.9)
Net income attributable to SCC	389.6	369.4	1,180.2	1,249.8
Capital investment	182.7	282.3	536.1	831.8
Property and mine development, net	9,395.3	9,311.8	9,395.3	9,311.8	$ 9,403.8
Total assets	16,494.4	14,316.7	16,494.4	14,316.7	$ 14,484.8
Intersegment sales
Financial information relating to segments
Net sales	(18.6)	(17.2)	(63.9)	(55.6)
Corporate, other and eliminations
Financial information relating to segments
Net sales	(18.6)	(17.2)	(63.9)	(55.6)
Cost of sales (exclusive of depreciation, amortization and depletion)	(16.9)	(18.8)	(70.3)	(60.5)
Selling, general and administrative	1.2	0.2	5.8	1.6
Depreciation, amortization and depletion	7.1	2.4	29.0	20.8
Exploration	2.2	2.3	2.5	3.5
Operating income	(12.2)	(3.3)	(30.9)	(21.0)
Capital investment	2.4	0.7	4.5	19.4
Property and mine development, net	337.8	553.3	337.8	553.3
Total assets	2,447.2	465.0	2,447.2	465.0
Reportable subsegments
Financial information relating to segments
Net sales	1,859.5	1,723.7	5,431.0	5,402.1
Mexican Open-pit \| Operating segment
Financial information relating to segments
Net sales	982.4	995.1	3,009.1	3,124.9
Cost of sales (exclusive of depreciation, amortization and depletion)	390.2	398.8	1,194.9	1,214.8
Selling, general and administrative	19.4	15.8	51.4	41.4
Depreciation, amortization and depletion	91.1	95.1	263.9	269.5
Exploration	0.5	0.4	1.4	1.5
Operating income	481.2	485.0	1,497.5	1,597.7
Capital investment	59.4	53.7	180.5	197.9
Property and mine development, net	4,691.2	4,579.5	4,691.2	4,579.5
Total assets	8,093.3	8,122.7	8,093.3	8,122.7
Mexican Open-pit \| Reportable subsegments \| Operating segment
Financial information relating to segments
Net sales	982.4	995.1	3,009.1	3,124.9
Mexican IMMSA Unit \| Operating segment
Financial information relating to segments
Net sales	106.7	107.3	350.1	409.6
Cost of sales (exclusive of depreciation, amortization and depletion)	80.7	74.7	285.3	271.2
Selling, general and administrative	2.0	1.9	6.0	6.8
Depreciation, amortization and depletion	13.7	11.5	39.9	33.6
Exploration	2.3	1.1	7.0	3.6
Operating income	8.0	18.1	11.9	94.4
Capital investment	32.1	10.2	79.6	32.6
Property and mine development, net	487.6	433.4	487.6	433.4
Total assets	839.3	974.1	839.3	974.1
Mexican IMMSA Unit \| Intersegment sales
Financial information relating to segments
Net sales	18.6	17.2	63.9	55.6
Mexican IMMSA Unit \| Reportable subsegments \| Operating segment
Financial information relating to segments
Net sales	88.1	90.1	286.2	354.0
Peruvian Operations \| Operating segment
Financial information relating to segments
Net sales	789.0	638.5	2,135.7	1,923.2
Cost of sales (exclusive of depreciation, amortization and depletion)	452.5	369.3	1,208.0	1,126.7
Selling, general and administrative	9.4	8.5	28.2	26.9
Depreciation, amortization and depletion	88.4	61.6	247.8	171.3
Exploration	1.9	2.2	9.0	11.7
Operating income	236.8	196.9	642.7	586.6
Capital investment	88.8	217.7	271.5	581.9
Property and mine development, net	3,878.7	3,745.6	3,878.7	3,745.6
Total assets	5,114.6	4,754.9	5,114.6	4,754.9
Peruvian Operations \| Reportable subsegments \| Operating segment
Financial information relating to segments
Net sales	$ 789.0	$ 638.5	$ 2,135.7	$ 1,923.2

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