Divestitures - Additional Information (Detail) $ in Millions	3 Months Ended		6 Months Ended	12 Months Ended
	Mar. 31, 2022 USD ($)	Mar. 31, 2021 USD ($) Employee t	Jun. 30, 2021 USD ($)	Dec. 31, 2021 USD ($)
Business Disposition [Line Items]
Net gain from asset sales	$ (1)	$ 27
Payments against the reserve	4	5		$ 23
Warrick Rolling Mill [Member]
Business Disposition [Line Items]
Net gain from asset sales		27
Warrick Rolling Mill [Member] | Held for Sale [Member]
Business Disposition [Line Items]
Total consideration		670
Assumption of other postretirement benefit liabilities		$ 69
Net gain from asset sales			$ 30
Remaining reserve				$ 70
Payments against the reserve	$ 2
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