Summary of Significant Accounting Policies - Additional Information (Detail)	3 Months Ended			12 Months Ended
	Mar. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2015 USD ($)	Dec. 31, 2017 USD ($) reporting_unit	Dec. 31, 2016 USD ($)	Dec. 31, 2015 USD ($)
Summary Of Significant Accounting Policies [Line Items]
Original maturity of cash equivalents				3 months
Number of reporting units | reporting_unit				8
Segment allocation, goodwill recognized		$ 4,535,000,000	$ 5,249,000,000	$ 4,535,000,000	$ 5,148,000,000	$ 5,249,000,000
Minimum percentage of estimated fair value of reporting unit to be less than carrying amount of goodwill				50.00%
Period required for impairment testing of reporting units				3 years
Goodwill impairment				$ 719,000,000	0	25,000,000
Businesses expected to be disposed within, years				1 year
Adjustments for New Accounting Pronouncement | Pro Forma
Summary Of Significant Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle, effect of change on operating results				$ 154,000,000	$ 135,000,000
Adjustments for New Accounting Pronouncement | Scenario, Forecast
Summary Of Significant Accounting Policies [Line Items]
New accounting pronouncement or change in accounting principle, effect of change on operating results	$ 20,000,000
Engineered Products and Solutions
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units | reporting_unit				4
Number of reporting units for goodwill allocation | reporting_unit				2
Number of reporting units being subjected to goodwill impairment test | reporting_unit				6
Goodwill impairment		719,000,000		$ 719,000,000
Engineered Products and Solutions | Fastening systems and rings
Summary Of Significant Accounting Policies [Line Items]
Segment allocation, goodwill recognized		2,221,000,000		2,221,000,000
Engineered Products and Solutions | Alcoa Power and Propulsion
Summary Of Significant Accounting Policies [Line Items]
Segment allocation, goodwill recognized		1,686,000,000		$ 1,686,000,000
Engineered Products and Solutions | Alcoa Forging and Extrusions
Summary Of Significant Accounting Policies [Line Items]
Goodwill impairment		$ 719,000,000
Transportation and Construction Solutions
Summary Of Significant Accounting Policies [Line Items]
Number of reporting units | reporting_unit				3
Goodwill impairment						25,000,000
Other Operating Segments
Summary Of Significant Accounting Policies [Line Items]
Qualitative assessment for reporting units | reporting_unit				1
Primary Metals
Summary Of Significant Accounting Policies [Line Items]
Goodwill impairment			$ 25,000,000
Implied fair value of goodwill						$ 0
Minimum | Engineered Products and Solutions
Summary Of Significant Accounting Policies [Line Items]
Percentage of total goodwill allocated to two reporting units				85.00%