RELATED PARTY TRANSACTIONS: (Details) (USD $) In Millions, unless otherwise specified	6 Months Ended					6 Months Ended			3 Months Ended			6 Months Ended		3 Months Ended			6 Months Ended			6 Months Ended			12 Months Ended	6 Months Ended						6 Months Ended			6 Months Ended			6 Months Ended			6 Months Ended
	Jun. 30, 2011	Jun. 30, 2010	Dec. 31, 2010	Jun. 30, 2011 Grupo Mexico, S.A.B de C.V. and affiliates	Dec. 31, 2010 Grupo Mexico, S.A.B de C.V. and affiliates	Jun. 30, 2011 Mexico Proyectos y Desarrollos, S.A. de C.V. and affiliates	Jun. 30, 2010 Mexico Proyectos y Desarrollos, S.A. de C.V. and affiliates	Dec. 31, 2010 Mexico Proyectos y Desarrollos, S.A. de C.V. and affiliates	Jun. 30, 2011 Asarco LLC	Mar. 31, 2011 Asarco LLC	Jun. 30, 2010 Asarco LLC	Jun. 30, 2011 Asarco LLC	Jun. 30, 2010 Asarco LLC	Jun. 30, 2010 Asarco LLC Cost of sales	Jun. 30, 2010 Asarco LLC Other income	Jun. 30, 2010 Asarco LLC Interest income	Jun. 30, 2011 Ferrocarril Mexicano, S.A. de C.V.	Jun. 30, 2010 Ferrocarril Mexicano, S.A. de C.V.	Dec. 31, 2010 Ferrocarril Mexicano, S.A. de C.V.	Jun. 30, 2011 Mexico Transportes Aereos S.A de C.V. ("Mextransport")	Jun. 30, 2010 Mexico Transportes Aereos S.A de C.V. ("Mextransport")	Dec. 31, 2010 Mexico Transportes Aereos S.A de C.V. ("Mextransport")	Dec. 31, 2007 Mexico Transportes Aereos S.A de C.V. ("Mextransport") Mexican subsidiaries	Jun. 30, 2011 Mexico Compania de Productos Automotrices, S.A. de C.V.	Jun. 30, 2010 Mexico Compania de Productos Automotrices, S.A. de C.V.	Jun. 30, 2011 Consorcio Tricobre	Jun. 30, 2010 Consorcio Tricobre	Dec. 31, 2010 Consorcio Tricobre	Jun. 30, 2011 Consorcio Tricobre Servicios de Ingenieria Consultec, S.A. de C. V.	Jun. 30, 2011 Higher Technology S.A.C.	Jun. 30, 2010 Higher Technology S.A.C.	Dec. 31, 2010 Higher Technology S.A.C.	Jun. 30, 2011 Breaker, S.A. de C.V	Jun. 30, 2010 Breaker, S.A. de C.V	Dec. 31, 2010 Breaker, S.A. de C.V	Jun. 30, 2011 Pigoba, S.A. de C.V	Jun. 30, 2010 Pigoba, S.A. de C.V	Dec. 31, 2010 Pigoba, S.A. de C.V	Jun. 30, 2011 Grupo Mexico Servicios, S.A de C.V	Jun. 30, 2010 Grupo Mexico Servicios, S.A de C.V	Jun. 30, 2011 Mexico Constructora Industrial S.A. de C.V	Jun. 30, 2011 Cia Perforadora Mexico S.A.P.I. de C.V and affiliates	Jun. 30, 2011 Servicios y Fabricaciones Mecanicas S.A.C.	Jun. 30, 2010 Servicios y Fabricaciones Mecanicas S.A.C.	Jun. 30, 2011 Sempertrans France Belting Technology	Jun. 30, 2010 Sempertrans France Belting Technology
Related party transactions
Spread on variable interest rate (as a percent)																				0.15%
Remaining balance at June 30, 2010	$ 2,750.4		$ 2,760.4																	$ 3.2
Amount recovered from previously written-off accounts receivable											7.7			5.0	1.6	1.1
Sales to related parties									34.6	17.9	5.1	52.5	5.1
Affiliate receivable	4.4		32.7	0.8	32.7	3.3			0.3			0.3
Affiliate payable	5.6		4.7	2.6	2.3			0.9									1.3		0.1	0.5		0.4		0.3				0.5				0.1	0.9		0.3			0.1
Total purchased from related parties	41.4	23.3				1.8	8.8					7.7					4.1	1.6		1.3	1.0			0.2	1.1	0.5	1.4			0.9	1.5		2.9	0.3		0.1	0.1		7.0	6.9	14.1	0.4	0.2	0.2	0.2	0.4
Ownership of Consorcio Tricobre held by Servicios de Ingenieria Consutec, S.A. de C. V. (as a percent)																													42.70%
Number of loans guaranteed by subsidiaries																							2
Original loan balance																				$ 8.5
Basis of variable interest rate																				Libor
Number of loans repaid																				1