SEGMENT AND RELATED INFORMATION: (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2016 USD ($) segment	Sep. 30, 2015 USD ($)	Dec. 31, 2015 USD ($)
Financial information related to segments
Number of reportable segments | segment			3
Financial information relating to segments
Net sales outside of segments	$ 1,400.7	$ 1,133.6	$ 3,980.8	$ 3,791.3
Cost of sales (exclusive of depreciation, amortization and depletion)	831.4	670.8	2,309.9	2,057.4
Selling, general and administrative	22.7	23.6	72.5	73.4
Depreciation, amortization and depletion	174.5	131.6	474.3	373.8
Exploration	9.7	13.7	30.4	36.2
Environmental remediation		7.0		23.5
Operating income	362.4	286.9	1,093.7	1,227.0
Interest, net	(71.3)	(61.9)	(214.1)	(136.2)
Other income (expense)	9.6	(4.3)	14.9	(9.6)
Income taxes	(111.2)	(125.3)	(305.4)	(411.6)
Equity earnings of affiliate	8.7	4.0	17.4	9.5
Non-controlling interest	(0.6)	(1.0)	(1.9)	(3.5)
Net income attributable to SCC	197.6	98.4	604.6	675.6
Capital investment	275.6	416.6	840.5	946.3
Property, net	8,581.4	7,872.9	8,581.4	7,872.9	$ 8,262.8
Total assets	13,063.0	12,732.3	13,063.0	12,732.3	$ 12,593.2
Payment to acquire business, net of cash acquired		100.4		100.4
Corporate, other and eliminations
Financial information relating to segments
Net sales outside of segments	(19.0)	(19.0)	(53.5)	(53.4)
Cost of sales (exclusive of depreciation, amortization and depletion)	(46.8)	(48.1)	(152.7)	(67.9)
Selling, general and administrative	0.3	(2.3)	1.0	2.9
Depreciation, amortization and depletion	10.2	(0.1)	5.4	1.6
Exploration	3.4	4.7	10.5	14.0
Operating income	13.9	26.8	82.3	(4.0)
Capital investment	2.3	96.2	2.4	104.0
Property, net	242.0	58.0	242.0	58.0
Total assets	(51.9)	1,215.1	(51.9)	1,215.1
Corporate, other and eliminations | El Pilar
Financial information relating to segments
Payment to acquire business, net of cash acquired		100.4		100.4
Mexican Open-pit | Operating segment
Financial information relating to segments
Net sales outside of segments	865.0	597.6	2,422.5	1,995.3
Cost of sales (exclusive of depreciation, amortization and depletion)	462.6	333.5	1,288.8	929.8
Selling, general and administrative	11.3	15.4	37.5	36.3
Depreciation, amortization and depletion	94.5	62.7	267.0	177.2
Exploration	2.5	3.4	5.5	5.2
Environmental remediation		7.0		23.5
Operating income	294.1	175.6	823.7	823.3
Capital investment	112.8	215.2	379.7	601.2
Property, net	5,094.2	4,824.7	5,094.2	4,824.7
Total assets	8,052.2	7,303.4	8,052.2	7,303.4
Mexican IMMSA Unit | Operating segment
Financial information relating to segments
Net sales outside of segments	94.9	68.4	247.5	245.8
Cost of sales (exclusive of depreciation, amortization and depletion)	73.8	80.3	215.8	247.8
Selling, general and administrative	2.1	1.5	5.4	4.9
Depreciation, amortization and depletion	15.8	9.0	39.6	24.6
Exploration	1.7	3.4	4.7	8.5
Operating income	20.5	(6.8)	35.5	13.4
Capital investment	8.7	14.0	26.2	29.4
Property, net	448.6	393.9	448.6	393.9
Total assets	800.3	815.8	800.3	815.8
Mexican IMMSA Unit | Intersegment sales
Financial information relating to segments
Net sales outside of segments	19.0	19.0	53.5	53.4
Peruvian Operations | Operating segment
Financial information relating to segments
Net sales outside of segments	440.8	467.6	1,310.8	1,550.2
Cost of sales (exclusive of depreciation, amortization and depletion)	341.8	305.1	958.0	947.7
Selling, general and administrative	9.0	9.0	28.6	29.3
Depreciation, amortization and depletion	54.0	60.0	162.3	170.4
Exploration	2.1	2.2	9.7	8.5
Operating income	33.9	91.3	152.2	394.3
Capital investment	151.8	91.2	432.2	211.7
Property, net	2,796.6	2,596.3	2,796.6	2,596.3
Total assets	$ 4,262.4	$ 3,398.0	$ 4,262.4	$ 3,398.0