Divestitures - Additional Information (Detail) $ in Millions	3 Months Ended		12 Months Ended
	Mar. 31, 2021 USD ($) Employee t	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)
Business Disposition [Line Items]
Net gain from asset sales	$ 27	$ 177
Elemental Environmental Solutions LLC [Member]
Business Disposition [Line Items]
Sale transaction value of waste processing		$ 250
Net gain from asset sales			$ 181
Net cash received			200
Escrow to be received for divestitures			50
Warrick Rolling Mill
Business Disposition [Line Items]
Net gain from asset sales	27
Warrick Rolling Mill | Held for Sale
Business Disposition [Line Items]
Sale transaction value of waste processing	670
Net gain from asset sales	27
Assets held for sale			648
Liabilities held for sale			$ 242
Assumption of other postretirement benefit liabilities	72
Transaction Costs	7
Estimated liabilities recorded	$ 70
Number of employees | Employee	1,150
Production of aluminum smelter and power plants (metric ton per year) | t	269,000
Number of employees | Employee	670