Other Financial Information - Additional Information (Detail) - San Ciprián Smelter [Member] - USD ($) $ in Millions	Jun. 30, 2025	Mar. 31, 2025
Other Non Operating Income Expense [Line Items]
Amount released from restricted cash		$ 11
Restricted cash available for capital improvements	$ 75

restricted cash available for capital improvements.

No definition available.

Name:	aa_RestrictedCashAvailableForCapitalImprovements
Namespace Prefix:	aa_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant