Derivatives and Other Financial Instruments - Schedule of Outstanding Quantities of Derivative Instruments (Detail) - Fair Value Inputs Level 1 and Level 2 [Member] kt in Thousands, € in Millions, ر.س in Millions, kr in Millions, R$ in Millions, MWh in Millions, $ in Millions	Jun. 30, 2026 EUR (€) MWh kt	Jun. 30, 2026 NOK (kr) MWh kt	Jun. 30, 2026 BRL (R$) MWh kt	Jun. 30, 2026 AUD ($) MWh kt	Jun. 30, 2026 SAR (ر.س) MWh kt	Jun. 30, 2025 EUR (€) MWh kt	Jun. 30, 2025 NOK (kr) MWh kt	Jun. 30, 2025 BRL (R$) MWh kt	Jun. 30, 2025 AUD ($) MWh kt	Jun. 30, 2025 SAR (ر.س) MWh kt
Foreign Exchange Buy Forwards [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Liabilities	€ 464	kr 193	R$ 18	$ 1,154		€ 176	kr 8	R$ 107	$ 821
Foreign Exchange Buy Forwards Undesignated [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Liabilities \| €	633					821
Foreign Exchange Sell Forwards Undesignated [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Liabilities \| €	0					32
Foreign Exchange Sell Forwards [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Liabilities \| €	€ 4					€ 10
Foreign Exchange Swap Undesignated [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Derivative Liabilities \| ر.س					ر.س 273					ر.س 0
Commodity Sell Forwards [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Outstanding quantities of derivative instruments \| kt	538	538	538	538	538	575	575	575	575	575
Outstanding alumina derivative instruments \| kt	68	68	68	68	68	0	0	0	0	0
Commodity Buy Forwards [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Outstanding quantities of derivative instruments \| kt	299	299	299	299	299	160	160	160	160	160
Commodity Buy Forwards Natural Gas [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Outstanding energy derivative instruments \| MWh	4	4	4	4	4	6	6	6	6	6
Commodity Buy Forwards Electricity [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis [Line Items]
Outstanding energy derivative instruments \| MWh	5	5	5	5	5	6	6	6	6	6