Divestitures - Additional Information (Detail) $ in Millions	3 Months Ended		9 Months Ended		12 Months Ended
	Jun. 30, 2021 USD ($)	Mar. 31, 2021 USD ($) Employee t	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2020 USD ($)
Business Disposition [Line Items]
Net gain from asset sales			$ 132	$ 174
Eastalco Aluminum Smelter Site [Member] | Maryland [Member]
Business Disposition [Line Items]
Total consideration	$ 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
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