Acquisitions and Divestitures - 2014 Acquisitions - Additional Information (Detail)		1 Months Ended		12 Months Ended
	Nov. 19, 2014 USD ($) $ / shares	Aug. 31, 2014 USD ($)	Jun. 30, 2014 USD ($) shares	Dec. 31, 2015 USD ($)	Dec. 31, 2014 USD ($) Employees Facilities	Dec. 31, 2013 USD ($)
Business Acquisition [Line Items]
Cash paid for business acquisition				$ 205,000,000	$ 2,458,000,000
Interest expenses				498,000,000	473,000,000	$ 453,000,000
Goodwill				5,401,000,000	5,247,000,000	3,415,000,000
Income tax expense				$ 445,000,000	$ 320,000,000	$ 428,000,000
Weighted average amortization period of intangible asset				35 years
Alcoa Fastening Systems and Rings [Member]
Business Acquisition [Line Items]
Goodwill				$ 1,117,000,000
Alcoa Forging and Extrusions [Member]
Business Acquisition [Line Items]
Goodwill				684,000,000
Firth Rixson [Member]
Business Acquisition [Line Items]
Business acquisition purchase price	$ 2,995,000,000		$ 2,850,000,000
Cash paid for business acquisition	2,385,000,000		2,350,000,000
Value of common stock issued	610,000,000		$ 500,000,000
Common stock issued as part of acquisition | shares			36,523,010
Business acquisition agreement date			Dec. 31, 2020
Business acquisition transaction cost	$ 35,000,000
Business acquisition common stock issued, price per share | $ / shares	$ 16.69
Professional fees and costs			$ 42,000,000
Professional fees and costs , after tax			34,000,000
Interest expenses			13,000,000
Interest expenses, after tax			8,000,000
Business acquisition, unsecured bridge term loan			2,500,000,000
Number of operating facilities | Facilities					13
Number of employees | Employees					2,400
Business acquisition sales					$ 81,000,000
After-tax operating income					$ (12,000,000)
Goodwill				1,801,000,000
Income tax expense				0
Business acquisition potential earn-out				$ 150,000,000
Business combination contingent consideration earnout period description				2019 through 2020
Firth Rixson [Member] | Maximum [Member]
Business Acquisition [Line Items]
Business acquisition potential earn-out			$ 150,000,000
Aluminum Brazing Sheet [Member]
Business Acquisition [Line Items]
Percentage of outstanding non controlling interest		30.00%
Amount of outstanding non controlling interest		$ 28,000,000
Difference between carrying value and fair value in acquisition of non-controlling interest		$ 3,000,000