Asset Retirement Obligations - Additional Information (Detail) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2025	Sep. 30, 2025
Kwinana Refinery [Member]
Asset Retirement Obligations [Line Items]
Liabilities incurred	$ 405	$ 405
Mine Areas [Member] | Australia [Member]
Asset Retirement Obligations [Line Items]
Liabilities incurred	78	78
Bauxite Residue Areas [Member] | Australia [Member]
Asset Retirement Obligations [Line Items]
Liabilities incurred	63	63
Pocos De Caldas Refinery [Member] | Brazil [Member]
Asset Retirement Obligations [Line Items]
Liabilities incurred	51	51
Estimated mine reclamations costs	2	2
Residue Area Closure and Water Treatment [Member]
Asset Retirement Obligations [Line Items]
Liabilities incurred	20	20
Spent Pot Lining Treatment and Disposal [Member]
Asset Retirement Obligations [Line Items]
Liabilities incurred	3	10
Non-Operating Bauxite Residue Areas [Member]
Asset Retirement Obligations [Line Items]
Liabilities incurred	$ 6	$ 6