Investments - Additional Information (Detail) $ in Millions	1 Months Ended			12 Months Ended
	Jun. 30, 2019	Jun. 30, 2018 USD ($)	Dec. 31, 2009	Dec. 31, 2020 USD ($)	Dec. 31, 2020 CAD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Jun. 30, 2018 CAD ($)
Schedule of Equity Method Investments [Line Items]
Dividends from equity investments				$ 44,000,000		$ 39,000,000	$ 45,000,000
Outstanding receivable for labor and other employee-related expenses				5,000,000		8,000,000
Combined investment in joint venture				1,041,000,000		1,103,000,000
Research and development expenses				$ 27,000,000		27,000,000	31,000,000
Maaden Alcoa Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Non-controlling interest, ownership percentage			25.10%	25.10%
Rio Tinto Plc [Member] | Elysis TM Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest percentage		48.235%
Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest in joint venture				25.10%	25.10%
Alcoa Corporation [Member] | Elysis TM Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest percentage		48.235%
Quebec Provincial Government [Member] | Elysis TM Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Limited partner ownership interest percentage		3.53%
Saudi Arabia [Member] | Ma’aden [Member] | Maaden Alcoa Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest in joint venture			74.90%	74.90%	74.90%
Ma'aden Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Joint venture shareholders agreement period, years				30 years	30 years
Joint venture shareholders agreement, automatic extension additional period, years				20 years	20 years
Ma'aden Joint Venture [Member] | Saudi Arabia [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest percentage transferred	25.10%
Alcoa Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Ownership interest in joint venture			25.10%
Equity interest	0.00%
Maaden Alcoa Joint Venture [Member]
Schedule of Equity Method Investments [Line Items]
Equity investments				$ 559,000,000		$ 603,000,000
Elysis TM Limited Partnership [Member]
Schedule of Equity Method Investments [Line Items]
Combined investment in joint venture		$ 147,000,000						$ 188
Contribution to joint venture							$ 5,000,000
Research and development expenses				3,000,000
Basis in investment, due to share of losses				0
Unrecognized losses				32,000,000
Elysis TM Limited Partnership [Member] | Alcoa Corporation [Member]
Schedule of Equity Method Investments [Line Items]
Contribution to joint venture				$ 14,000,000	$ 18
Elysis TM Limited Partnership [Member] | Alcoa Corporation And Rio Tinto Plc
Schedule of Equity Method Investments [Line Items]
Commitment to invest In joint venture		$ 43,000,000						$ 55