Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2020 USD ($) Reporting_Unit	Jan. 01, 2020	Dec. 31, 2019 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Original maturity of cash equivalents	three months
Segment Allocation, Goodwill Recognized | $	$ 145		$ 150
Weighted average yield curve duration discount rate	11 years
Defined benefit plan, description of basis used to determine overall expected long-term rate-of-return on assets assumption	The expected long-term rate of return on plan assets is generally applied to a five-year market-related value of plan assets (a four-year average or the fair value at the plan measurement date is used for certain non-U.S. plans).
Maximum hedging contracts period, in years	5 years
Accounting Standards Update 2019-08 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect		true
Accounting Standards Update 2018-15 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect		true
Accounting Standards Update 2018-14 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect		true
Accounting Standards Update 2018-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect		true
Accounting Standards Update 2016-13 [Member]
Summary Of Significant Accounting Policies [Line Items]
Change in accounting principle, accounting standards update, adopted		true
Change in accounting principle, accounting standards update, adoption date		Jan. 01, 2020
Change in accounting principle, accounting standards update, immaterial effect		true
Alcoa Corporation [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation	5
Alcoa Corporation [Member] | Bauxite [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation	1
Segment Allocation, Goodwill Recognized | $	$ 49
Alcoa Corporation [Member] | Alumina [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation	1
Segment Allocation, Goodwill Recognized | $	$ 96
Alcoa Corporation [Member] | Aluminum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation	3
Minimum [Member] | Bauxite Mining [Member]
Summary Of Significant Accounting Policies [Line Items]
Period of mining	1 year
Maximum [Member] | Bauxite Mining [Member]
Summary Of Significant Accounting Policies [Line Items]
Period of mining	5 years