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Restructuring and Other Charges, Net - Additional Information (Detail) - USD ($)
$ in Millions
3 Months Ended 12 Months Ended
Mar. 31, 2021
Mar. 31, 2020
Dec. 31, 2020
Dec. 31, 2019
Restructuring Cost and Reserve [Line Items]        
Restructuring and other charges, net (D) $ 7 $ 2    
Curtailment of certain pension benefits   $ 3    
Restructuring reserve balance 56   $ 63 $ 137
Payments for restructuring 13   120  
Noncurrent portion of the reserve 1   1  
Aviles and La Coruna Smelters [Member]        
Restructuring Cost and Reserve [Line Items]        
Restructuring reserve balance     $ 30  
Payments for restructuring 6      
Severance and Exit Costs [Member] | Warrick Rolling Mill [Member]        
Restructuring Cost and Reserve [Line Items]        
Settlement and curtailment of certain other postretirement benefits 9      
Contract Termination [Member] | Wenatchee (Washington) and Intalco (Washington) [Member]        
Restructuring Cost and Reserve [Line Items]        
Restructuring and other charges, net (D) 6      
Permanent Close [Member] | Tennessee [Member]        
Restructuring Cost and Reserve [Line Items]        
Restructuring and other charges (reversal) (12)      
Other Item Charges [Member]        
Restructuring Cost and Reserve [Line Items]        
Restructuring and other charges, net (D) $ 4