v3.26.1
Derivatives and Other Financial Instruments - Additional Information (Detail)
kt in Thousands, $ in Millions
 3 Months Ended 6 Months Ended
Jun. 30, 2026
USD ($)
kt
Mar. 31, 2026
USD ($)
Jun. 30, 2025
USD ($)
Mar. 31, 2025
USD ($)
Jun. 30, 2026
USD ($)
Member
kt
Jun. 30, 2025
USD ($)
Dec. 31, 2025
kt
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Minimum members required for strategic risk management committee | Member         3    
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings   $ (331)   $ (175) $ (190) $ (80)  
Level 1 [Member] | Spain [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Derivative instruments, expiration month and year         2027-12    
Level 1 Derivative Instruments [Member] | Level 1 [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Derivative instruments, expiration month and year         2026-12    
Level 1 and 2 Derivative Instruments [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings   $ (70)   $ (4) $ (45) (2)  
Commodity Contract | Level 1 [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Derivative instruments, expiration month and year         2031-12    
Commodity Contract | Level 1 [Member] | Spain [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Derivative instruments, expiration month and year         2027-12    
Foreign Exchange Forward | Norway [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Derivative instruments, expiration month and year         2027-12    
Foreign Exchange Forward | Norway [Member] | Euro Power Purchases [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Derivative instruments, expiration month and year         2031-12    
Foreign Exchange Forward | Brazil [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Derivative instruments, expiration month and year         2026-10    
Foreign Exchange Forward | Australia [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Derivative instruments, expiration month and year         2032-12    
Foreign Exchange Swap | Saudi Riyals [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Derivative instruments, expiration month and year         2026-08    
Energy Contracts [Member] | Spain [Member] | Natural Gas Purchases [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Derivative instruments, expiration month and year         2027-12    
Derivatives Designated as Hedging Instruments [Member] | Level 1 and 2 Derivative Instruments [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings $ (45)       $ (70) (4)  
Derivatives Designated as Hedging Instruments [Member] | Power Contract [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Aluminum forecast sales | kt 893       893   1,005
Derivatives Designated as Hedging Instruments [Member] | Power Contract [Member] | Cash Flow Hedging [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Amount of (loss) gain expected to be recognized into earnings over the next 12 months         $ (376)    
Derivatives Designated as Hedging Instruments [Member] | Sales [Member] | Level 1 and 2 Derivative Instruments [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings $ (39)       (53) (2)  
Derivatives Designated as Hedging Instruments [Member] | Cost of Goods Sold [Member] | Level 1 and 2 Derivative Instruments [Member]              
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]              
Realized gain (loss) reclassified from Accumulated other comprehensive loss to earnings $ (6)   $ (2)   $ (17) $ (2)