Divestitures - Additional Information (Detail) $ in Millions	1 Months Ended	3 Months Ended		6 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($)	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($) Employee t	Jun. 30, 2021 USD ($)	Jun. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)
Business Disposition [Line Items]
Net gain from asset sales				$ 124	$ 176
Eastalco Aluminum Smelter Site [Member] | Maryland [Member]
Business Disposition [Line Items]
Total consideration	$ 100	$ 100		100
Net gain from asset sales	90
Net cash received	94
Net gain from asset sales after tax	89
Warrick Rolling Mill [Member] | Held for Sale [Member]
Business Disposition [Line Items]
Total consideration			$ 670
Net gain from asset sales		3	27
Assets held for sale						$ 648
Liabilities held for sale						242
Assumption of other postretirement benefit liabilities	$ 66	66		$ 66
Decrease in other postretirement benefit liabilities after post closing adjustment		6
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
Elemental Environmental Solutions LLC [Member]
Business Disposition [Line Items]
Total consideration							$ 250
Net gain from asset sales						181
Net cash received						200
Escrow to be received for divestitures						$ 50