Summary of Significant Accounting Policies - Additional Information (Detail) $ in Millions	12 Months Ended
	Dec. 31, 2019 USD ($) Reporting_Unit	Dec. 31, 2018 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Original maturity of cash equivalents	three months
Segment Allocation, Goodwill Recognized | $	$ 150	$ 151
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
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 | $	$ 50
Alcoa Corporation [Member] | Alumina [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation	1
Segment Allocation, Goodwill Recognized | $	$ 100
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