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Debt - Additional Information (Detail) ¥ in Millions	1 Months Ended		3 Months Ended			6 Months Ended
Debt - Additional Information (Detail) ¥ in Millions	Apr. 30, 2025 USD ($)	Mar. 31, 2025 USD ($)	Jun. 30, 2025 USD ($)	Mar. 31, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2025 USD ($)	Jun. 30, 2024 USD ($)	Jun. 30, 2024 JPY (¥)	Dec. 31, 2024 USD ($)	Mar. 31, 2024
Debt Instrument [Line Items]
Short-term borrowings			$ 8,000,000			$ 8,000,000			$ 50,000,000
Borrowings of inventory related to agreement			7,000,000		$ 24,000,000	51,000,000	$ 45,000,000
Repurchase of inventory related to agreement			44,000,000		45,000,000	$ 93,000,000	70,000,000
Extended maturity date of credit facility	2025-11
Other Long Term Debt [Member]
Debt Instrument [Line Items]
Amounts borrowed under the credit facility	$ 74,000,000
Alcoa Nederland Holding BV [Member] \| 5.500% Notes, due 2027 [Member]
Debt Instrument [Line Items]
Principal amount of debt		$ 750,000,000		$ 750,000,000
Senior notes, interest percentage		5.50%		5.50%
Debt instrument maturity date		2027		2027
Alcoa Nederland Holding BV [Member] \| 6.125% Notes, due 2028 [Member]
Debt Instrument [Line Items]
Principal amount of debt		$ 500,000,000		$ 500,000,000
Senior notes, interest percentage		6.125%		6.125%
Debt instrument maturity date		2028		2028
Alcoa Nederland Holding BV [Member] \| 7.125% Notes, due 2031 [Member]
Debt Instrument [Line Items]
Senior notes, interest percentage										7.125%
Alumina Pty Ltd [Member] \| 6.125% Notes, due 2030 [Member]
Debt Instrument [Line Items]
Principal amount of debt		$ 500,000,000		$ 500,000,000
Proceeds from issuance of public debt offering		$ 985,000,000
Senior notes, interest percentage		6.125%		6.125%
Debt instrument maturity date		2030		2030
Debt instrument, frequency of periodic payment						semi-annually
Debt instrument, date of first required payment		Sep. 15, 2025
Alumina Pty Ltd [Member] \| 6.125% Notes, due 2030 [Member] \| After March 15, 2027 [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt instrument redemption period						10 days
Alumina Pty Ltd [Member] \| 6.125% Notes, due 2030 [Member] \| After March 15, 2027 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt instrument redemption period						60 days
Debt instrument redemption price percentage						103.063%
Alumina Pty Ltd [Member] \| 6.125% Notes, due 2030 [Member] \| Change in Control [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt instrument redemption price percentage						101.00%
Alumina Pty Ltd [Member] \| 6.375% Notes, due 2032 [Member]
Debt Instrument [Line Items]
Principal amount of debt		$ 500,000,000		$ 500,000,000
Proceeds from issuance of public debt offering		$ 985,000,000
Senior notes, interest percentage		6.375%		6.375%
Debt instrument maturity date		2032		2032
Debt instrument, frequency of periodic payment						semi-annually
Debt instrument, date of first required payment		Sep. 15, 2025
Alumina Pty Ltd [Member] \| 6.375% Notes, due 2032 [Member] \| After March 15, 2027 [Member] \| Minimum [Member]
Debt Instrument [Line Items]
Debt instrument redemption period						10 days
Alumina Pty Ltd [Member] \| 6.375% Notes, due 2032 [Member] \| After March 15, 2027 [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt instrument redemption period						60 days
Debt instrument redemption price percentage						103.188%
Alumina Pty Ltd [Member] \| 6.375% Notes, due 2032 [Member] \| Change in Control [Member] \| Maximum [Member]
Debt Instrument [Line Items]
Debt instrument redemption price percentage						101.00%
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Principal amount of debt			0			$ 0			0
Amounts borrowed under the credit facility			0		0	0	0
Unsecured revolving credit facility			1,250,000,000			1,250,000,000
Japanese Yen Revolving Credit Facility [Member] \| Alcoa Nederland Holding BV [Member]
Debt Instrument [Line Items]
Principal amount of debt			0			0			$ 0
Amounts borrowed under the credit facility			$ 0		$ 0	$ 0	201,000,000	¥ 29,686
Amounts repaid under the credit facility							$ 196,000,000	¥ 29,686
Unsecured revolving credit facility	$ 200,000,000	$ 250,000,000		$ 250,000,000
Extended maturity date of credit facility	2026-04
Tender Offers [Member] \| Alcoa Nederland Holding BV [Member] \| 5.500% Notes, due 2027 [Member]
Debt Instrument [Line Items]
Principal amount of debt		750,000,000		750,000,000
Settlement of debt		609,000,000
Debt settlement expenses				12,000,000
Tender Offers [Member] \| Alcoa Nederland Holding BV [Member] \| 6.125% Notes, due 2028 [Member]
Debt Instrument [Line Items]
Principal amount of debt		500,000,000		500,000,000
Settlement of debt		$ 281,000,000
Debt settlement expenses				$ 12,000,000