Acquisitions and Divestitures - 2014 Divestiture - Additional Information (Detail) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2014 USD ($)	Dec. 31, 2014 USD ($) Employees Mills t	Dec. 31, 2013 USD ($) Employees t
Business Acquisition [Line Items]
Loss from divestitures before income tax	$ 332
Net loss , after tax and noncontrolling interest	$ 163
Percent of equity investments in other entity	40.00%	40.00%
Aluminum Rod Plant [Member]
Business Acquisition [Line Items]
Sale generated in last annual period prior to divestiture			$ 200
Number of employees | Employees			60
2014 Divestitures [Member]
Business Acquisition [Line Items]
Net cash proceeds		$ 247
Loss from divestitures before income tax		332
Net loss , after tax and noncontrolling interest		$ 163
Alcoa [Member]
Business Acquisition [Line Items]
Percent of equity investments in other entity	60.00%	60.00%
Alcoa World Alumina and Chemicals [Member]
Business Acquisition [Line Items]
Sale generated in last annual period prior to divestiture		$ 200
Number of employees | Employees		500
Percent of equity investments in other entity	55.00%	55.00%
Plant capacity, in metric tons per year | t		778,800
Mt Holly Smelter [Member]
Business Acquisition [Line Items]
Sale generated in last annual period prior to divestiture			$ 280
Number of employees | Employees			250
Percent of equity investments in other entity	50.33%	50.33%
Plant capacity, in metric tons per year | t			115,000
Atlas Holdings Llc [Member]
Business Acquisition [Line Items]
Sale generated in last annual period prior to divestiture			$ 500
Number of employees | Employees			750
Number of rolling mills | Mills		3