Acquisitions and Divestitures - Additional Information (Detail) $ in Millions		1 Months Ended			12 Months Ended
	Nov. 01, 2016 USD ($)	Nov. 30, 2017 USD ($)	Jun. 30, 2017 USD ($)	Feb. 28, 2017 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($) Employees HydroelectricPowerDevelopments	Dec. 31, 2014
Business Disposition [Line Items]
Cash provided at separation to Parent Company	$ 1,072				$ 247	$ 1,072
Other current liabilities					412	$ 707
Alcoa Corporation [Member]
Business Disposition [Line Items]
Percent of equity investments in other entity							60.00%
AMJ [Member]
Business Disposition [Line Items]
Percent of equity investments in other entity							55.00%
Yadkin [Member]
Business Disposition [Line Items]
Number of hydroelectric power developments | HydroelectricPowerDevelopments						4
Net cash received		$ 8	$ 8	$ 249
Cash provided at separation to Parent Company						$ 243
Sale generated						$ 29
Number of employees | Employees						30
Income (loss) from divestitures before income tax					122
Income (loss) from divestitures after income tax					$ 122
Other current liabilities						$ 243