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Debt - Additional Information (Detail) ¥ in Millions	1 Months Ended		3 Months Ended
Debt - Additional Information (Detail) ¥ in Millions	Apr. 30, 2025 USD ($)	Mar. 31, 2025 USD ($)	Mar. 31, 2025 USD ($)	Mar. 31, 2024 USD ($)	Mar. 31, 2024 JPY (¥)	Dec. 31, 2024 USD ($)
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Short-term borrowings		$ 45,000,000	$ 45,000,000			$ 50,000,000
Borrowings of inventory related to agreement			44,000,000	$ 21,000,000
Repurchase of inventory related to agreement			49,000,000	$ 25,000,000
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%	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%
Subsequent Event [Member]
Debt Instrument [Line Items]
Extended maturity date of credit facility	2025-11
Subsequent Event [Member] \| Other Long Term Debt [Member]
Debt Instrument [Line Items]
Amounts borrowed under the credit facility	$ 74,000,000
Revolving Credit Facility [Member]
Debt Instrument [Line Items]
Principal amount of debt		$ 0	$ 0			0
Amounts borrowed under the credit facility			0	$ 0
Unsecured revolving credit facility		1,250,000,000	1,250,000,000
$250 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	201,000,000	¥ 29,686
Amounts repaid under the credit facility				$ 196,000,000	¥ 29,686
Unsecured revolving credit facility		250,000,000	250,000,000
Extended maturity date of credit facility	2026-04
$250 Japanese Yen Revolving Credit Facility [Member] \| Subsequent Event [Member] \| Alcoa Nederland Holding BV [Member]
Debt Instrument [Line Items]
Unsecured revolving credit facility	$ 200,000,000
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