Summary of Significant Accounting Policies - Additional Information (Detail)	12 Months Ended
	Dec. 31, 2017 USD ($) Reporting_Unit	Dec. 31, 2016 USD ($) Reporting_Unit	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Original maturity of cash equivalents	3 months
Segment Allocation, Goodwill Recognized	$ 989,000,000	$ 989,000,000
Minimum percentage of estimated fair value of reporting unit to be less than carrying amount of goodwill	50.00%
Goodwill impairment	$ 0
Maximum hedging contracts period, in years	5 years
Cost of goods sold	$ 9,072,000,000	7,898,000,000	$ 9,039,000,000
Selling, general administrative, and other expenses	284,000,000	$ 359,000,000	353,000,000
Accounting Standards Update 2017-07 [Member]
Summary Of Significant Accounting Policies [Line Items]
Cost of goods sold	81
Selling, general administrative, and other expenses	4
Other (income) expenses, net	58
Other (income) expenses, net	27
Aluminum [Member]
Summary Of Significant Accounting Policies [Line Items]
Segment Allocation, Goodwill Recognized	$ 0
Maximum [Member] | Bauxite Mining [Member]
Summary Of Significant Accounting Policies [Line Items]
Period of mining	5 years
Minimum [Member] | Bauxite Mining [Member]
Summary Of Significant Accounting Policies [Line Items]
Period of mining	1 year
Alcoa Corporation [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation | Reporting_Unit	5	6
Cost of goods sold		$ 40,000,000	93,000,000
Selling, general administrative, and other expenses		$ 150,000,000	$ 146,000,000
Alcoa Corporation [Member] | Bauxite [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation | Reporting_Unit	1
Segment Allocation, Goodwill Recognized	$ 51,000,000
Alcoa Corporation [Member] | Alumina [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation | Reporting_Unit	1
Segment Allocation, Goodwill Recognized	$ 103,000,000
Alcoa Corporation [Member] | Aluminum [Member]
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units for goodwill allocation | Reporting_Unit	3