RELATED PARTY TRANSACTIONS: (Details) $ in Millions	9 Months Ended		12 Months Ended
	Sep. 30, 2024 USD ($)	Sep. 30, 2023 USD ($)	Dec. 31, 2020 MWh	Dec. 31, 2012 item MW	Dec. 31, 2023 USD ($)	Dec. 31, 2014 item
RELATED PARTY TRANSACTIONS:
Related parties receivable current:	$ 12.3				$ 27.3
Affiliates
RELATED PARTY TRANSACTIONS:
Related parties receivable current:	12.3				27.3
Other accrued liabilities	54.8				93.6
Total purchases	267.2	$ 322.5
Total sales	$ 2.3	2.1
Affiliates | Equity investment in affiliate
RELATED PARTY TRANSACTIONS:
Ownership percentage	44.20%
Affiliates | Apu Coropuna S.R.L.
RELATED PARTY TRANSACTIONS:
Ownership percentage	30.00%
Affiliates | Minimum
RELATED PARTY TRANSACTIONS:
Aggregate consideration of Material Affiliate Transaction to be be authorized by the General Counsel and CFO	$ 8.0
Affiliates | Maximum
RELATED PARTY TRANSACTIONS:
Aggregate consideration of Material Affiliate Transaction to be be authorized by the General Counsel and CFO	10.0
Related party receivable and payable	0.1				0.1
Total sales	0.1	0.1
Threshold revenue from related parties	0.1	0.1
Grupo Mexico and affiliates | Affiliates
RELATED PARTY TRANSACTIONS:
Total sales	51.1	80.8
Asarco LLC ("Asarco") | Affiliates
RELATED PARTY TRANSACTIONS:
Related parties receivable current:	10.4				9.4
Other accrued liabilities	15.9				13.8
Total purchases	3.7	31.4
Total sales	27.0	29.9
AMMINCO Apoyo Administrativo, S.A. de C.V. ("AMMINCO") | Affiliates
RELATED PARTY TRANSACTIONS:
Other accrued liabilities	7.3				5.0
Total purchases	7.5	7.5
Total sales	0.1
Eolica El Retiro, S.A.P.I. de C.V.
RELATED PARTY TRANSACTIONS:
Number of wind turbines | item						37
Eolica El Retiro, S.A.P.I. de C.V. | Affiliates
RELATED PARTY TRANSACTIONS:
Other accrued liabilities	0.2				0.2
Total purchases	$ 4.4	$ 2.1
Percentage of supply to third-party energy users	30.90%	14.90%
Ferrocarril Mexicano, S.A. de C.V. | Affiliates
RELATED PARTY TRANSACTIONS:
Other accrued liabilities	$ 5.4				7.9
Total purchases	37.2	$ 40.1
Grupo Mexico | Affiliates
RELATED PARTY TRANSACTIONS:
Total purchases	2.7	3.0
Grupo Mexico Servicios, S.A. de C.V. | Affiliates
RELATED PARTY TRANSACTIONS:
Other accrued liabilities	1.8				2.7
Total purchases	15.1	15.1
Donations	1.9	3.4
MGE | Mexico
RELATED PARTY TRANSACTIONS:
Number of natural gas-fired combined cycle power generating units | item				2
MGE | Affiliates
RELATED PARTY TRANSACTIONS:
Related parties receivable current:	0.8				17.1
Other accrued liabilities	9.2				50.3
Total purchases	123.4	176.9
Total sales	$ 24.0	$ 50.9
MGE | Affiliates | Mexico
RELATED PARTY TRANSACTIONS:
Net total capacity (in megawatts) | MW				516.2
Percentage of supply to third-party energy users	8.00%	8.20%
Mexico Compania Constructora S.A de C.V. | Affiliates
RELATED PARTY TRANSACTIONS:
Other accrued liabilities	$ 7.4				9.5
Total purchases	58.0	$ 37.4
Parque Eolico de Fenicias, S. de R.L. de C.V. | Affiliates
RELATED PARTY TRANSACTIONS:
Other accrued liabilities	5.3
Total purchases	$ 4.6
Purchase agreement, contracted power capacity (in megawatts per year) | MWh			611,400
Term of power purchase agreement related to sale of power plant			20 years
Percentage of supply to third-party energy users	36.20%
Promotora Infraestructura Renovable Mex S.A. de C.V. | Affiliates
RELATED PARTY TRANSACTIONS:
Other accrued liabilities	$ 0.1
Grupo Mexico Servicios de Ingenieria, S.A. de C.V. | Affiliates
RELATED PARTY TRANSACTIONS:
Other accrued liabilities	0.9				3.5
Total purchases	13.3	12.0
Boutique Bowling de Mexico, S.A. de C.V. | Affiliates
RELATED PARTY TRANSACTIONS:
Other accrued liabilities	0.5				0.3
Total purchases	0.4	0.5
Total sales	0.1	0.1
Empresarios Industriales de Mexico, S.A. de C.V | Affiliates
RELATED PARTY TRANSACTIONS:
Related parties receivable current:	0.8				0.6
Total sales	0.3	0.2
Mexico Transportes Aereos S.A. de C.V. ("Mextransport") | Affiliates
RELATED PARTY TRANSACTIONS:
Related parties receivable current:	0.2				0.1
Other accrued liabilities	0.5				0.3
Total purchases	2.1	2.2
Total sales	1.8	1.7
Operadora de Cinemas S.A. de C.V. | Affiliates
RELATED PARTY TRANSACTIONS:
Related parties receivable current:	0.1				0.1
Other accrued liabilities	0.3				$ 0.1
Total purchases	0.2	0.3
Total sales	$ 0.1	$ 0.1