Fair Value Measurements - Derivative Assets and Liabilities on Recurring Basis (Details) - Recurring - USD ($) $ in Millions	Jun. 30, 2026	Mar. 31, 2026
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Derivative Asset, Master Netting Adjustment	$ (206)	$ (188)
Fair value of derivative, asset	218	57
Derivative Liability, Master Netting Adjustment	206	188
Derivative Liability	(134)	(642)
Level 2 Instruments
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	424	245
Liabilities	(340)	(830)
Level 2 Instruments \| Aluminum Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	380	197
Liabilities	(270)	(767)
Level 2 Instruments \| Currency Exchange Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	35	31
Liabilities	(67)	(57)
Level 2 Instruments \| Energy Contracts
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	9	17
Liabilities	(2)	(3)
Level 2 Instruments \| Interest Rate Swap
Fair Value, Assets and Liabilities Measured on Recurring and Nonrecurring Basis [Abstract]
Assets	0	0
Liabilities	$ (1)	$ (3)