RELATED PARTY TRANSACTIONS: (Details) MXN in Millions, $ in Millions	1 Months Ended	3 Months Ended	9 Months Ended			12 Months Ended
	May 31, 2010 USD ($)	Mar. 31, 2012 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2015 USD ($)	Sep. 30, 2014 USD ($)	Dec. 31, 2012 USD ($) item MW	Dec. 31, 2005 item	Dec. 31, 2015 USD ($)	Aug. 04, 2014 item	Mar. 31, 2012 MXN	Mar. 31, 2012 USD ($)
RELATED PARTY TRANSACTIONS:
Related parties receivable current:			$ 40.6					$ 15.8
Related parties receivable non-current:								111.2
Related parties payable:			70.0					69.3
Asarco LLC
RELATED PARTY TRANSACTIONS:
Related parties receivable current:			7.7
Related parties payable:			37.8					20.6
Total purchases			26.9	$ 23.9
Total sales			37.0	71.7
Compania Perforadora Mexico S.A.P.I. de C.V. and affiliates
RELATED PARTY TRANSACTIONS:
Related parties receivable current:			1.3					0.7
Total purchases				0.3
Total sales			0.5	0.4
Eolica El Retiro, S.A.P.I. de C.V.
RELATED PARTY TRANSACTIONS:
Related parties payable:			0.1					0.1
Total purchases			$ 0.8	7.1
Percentage of supply to third-party energy users			11.00%
Number of wind turbines | item									37
Ferrocarril Mexicano, S.A. de C.V.
RELATED PARTY TRANSACTIONS:
Related parties receivable current:								0.2
Related parties payable:			$ 0.1
Total purchases			33.9	17.8
Grupo Mexico
RELATED PARTY TRANSACTIONS:
Related parties receivable current:			4.9					0.6
Related parties payable:			1.4					12.0
Total purchases			15.0	10.4
Total sales			0.6		$ 0.1
Mexico Generadora de Energia S. de R.L. ("MGE")
RELATED PARTY TRANSACTIONS:
Related parties receivable current:			25.3					13.9
Related parties receivable non-current:						$ 184.0		111.2
Related parties payable:			18.4					23.0
Total purchases			171.3	97.1
Total sales			72.6	64.3
Maximum amount of line of credit granted to related party	$ 350.0
Interest rate (as a percent)	4.40%					5.75%
Loan repaid		$ 150.0
Interest earned			4.2	7.0
Percentage of supply to third-party energy users						12.00%
Mexico Generadora de Energia S. de R.L. ("MGE") | Mexico
RELATED PARTY TRANSACTIONS:
Number of power plants | item						2	2
Net total capacity (in megawatts) | MW						516.2
Mexico Generadora de Energia S. de R.L. ("MGE") | Controladora de Infraestructura Energetica Mexico, S. A. de C. V.
RELATED PARTY TRANSACTIONS:
Percentage of interest acquired										99.999%	99.999%
Value of interest acquired										MXN 1,928.6	$ 150.0
Mexico Generadora de Energia S. de R.L. ("MGE") | Controladora de Infraestructura Energetica Mexico, S. A. de C. V. | Mexico
RELATED PARTY TRANSACTIONS:
Percentage of interest acquired						99.999%
Mexico Generadora de Energia S. de R.L. ("MGE") | Minera Mexico | Maximum
RELATED PARTY TRANSACTIONS:
Ownership percentage										0.001%	0.001%
Mexico Proyectos y Desarrollos, S.A. de C.V. and affiliates
RELATED PARTY TRANSACTIONS:
Related parties receivable current:			1.2					0.3
Related parties payable:			9.8					11.8
Total purchases			47.1	44.2
Total sales			0.3	0.7
Boutique Bowling de Mexico S.A. de C.V.
RELATED PARTY TRANSACTIONS:
Related parties payable:								0.2
Total purchases			0.3
Total sales			0.2
Mexico Transportes Aereos S.A. de C.V. ("Mextransport")
RELATED PARTY TRANSACTIONS:
Related parties payable:			1.0					0.5
Total purchases			1.5	1.1
Total sales			0.4	0.3
Operadora de Generadoras de Energia Mexico S.A. de C.V.
RELATED PARTY TRANSACTIONS:
Related parties receivable current:								0.1
Operadora de Cinemas S.A. de C.V.
RELATED PARTY TRANSACTIONS:
Related parties receivable current:			0.2
Related parties payable:			0.3					0.2
Total purchases			0.5
Total sales			0.1
Breaker, S.A. de C.V and affiliates ("Breaker")
RELATED PARTY TRANSACTIONS:
Related parties payable:			0.3					0.3
Total purchases			0.5	3.8
Higher Technology S.A.C.
RELATED PARTY TRANSACTIONS:
Related parties payable:			0.6
Total purchases			1.0	1.1
Sempertrans and affiliates
RELATED PARTY TRANSACTIONS:
Related parties payable:								$ 0.6
Total purchases				0.5
Pigoba S.A. de C.V.
RELATED PARTY TRANSACTIONS:
Total purchases			0.1
Servicios y Fabricaciones Mecanicas S.A.C.
RELATED PARTY TRANSACTIONS:
Related parties payable:			0.2
Total purchases			$ 0.4	0.5
Equity investment in affiliate
RELATED PARTY TRANSACTIONS:
Ownership percentage			44.20%
Grupo Mexico and affiliates
RELATED PARTY TRANSACTIONS:
Total purchases			$ 295.0	200.8
Total sales			111.0	137.2
Companies with relationship to SCC executive officers' families
RELATED PARTY TRANSACTIONS:
Total purchases			2.0	5.9
Companies with relationship to the controlling group
RELATED PARTY TRANSACTIONS:
Total purchases			2.3	1.1
Total sales			$ 0.7	$ 0.3