Fair Value Measurements - Derivative Assets and Liabilities on Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Dec. 31, 2025	Mar. 31, 2025
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Derivative Asset, Master Netting Adjustment	$ (78)	$ (59)
Fair value of derivative, asset	36	122
Derivative Liability, Master Netting Adjustment	78	59
Derivative Liability	(313)	(54)
Level 2 Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	114	181
Liabilities	(391)	(113)
Level 2 Instruments \| Aluminum Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	83	152
Liabilities	(369)	(62)
Level 2 Instruments \| Currency Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	30	23
Liabilities	(14)	(45)
Level 2 Instruments \| Energy Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	1	6
Liabilities	(3)	(1)
Level 2 Instruments \| Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	0	0
Liabilities	$ (5)	$ (5)