Financial Instruments and Commodity Contracts (Gain (Loss) Reclassification Summarization) (Details) - USD ($) $ in Millions	12 Months Ended
Mar. 31, 2025	Mar. 31, 2024	Mar. 31, 2023
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	$ 4	$ 1	$ 4
Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0	1	4
Aluminium Contracts [Member] \| Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0	0	0
Aluminium Contracts [Member] \| Net sales \| Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) reclassified from AOCI into income (expense)	(185)	181	332
Aluminium Contracts [Member] \| Cost of goods sold \| Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) reclassified from AOCI into income (expense)	(1)	5	0
Energy Related Derivative [Member] \| Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) reclassified from AOCI into income (expense)	(8)	(5)	32
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0	0	0
Foreign Exchange Contract \| Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	0	1	4
Foreign Exchange Contract \| Net sales \| Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) reclassified from AOCI into income (expense)	(21)	(19)	(57)
Foreign Exchange Contract \| Cost of goods sold \| Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) reclassified from AOCI into income (expense)	(9)	17	18
Foreign Exchange Contract \| Selling, General and Administrative Expenses \| Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) reclassified from AOCI into income (expense)	0	0	1
Foreign Exchange Contract \| Depreciation and amortization \| Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) reclassified from AOCI into income (expense)	(3)	$ (3)	$ (5)
Foreign Exchange Contract \| Other expense (income), net \| Cash flow hedges
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Instruments, Gain (Loss) Recognized in Income, Ineffective Portion and Amount Excluded from Effectiveness Testing, Net	$ 4

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4E
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4E

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The effective portion of net gain (loss) reclassified from accumulated other comprehensive income into income on derivative instruments designated and qualifying as hedging instruments.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Name Accounting Standards Codification
-Section 50
-Paragraph 4D
-Subparagraph (b)
-SubTopic 10
-Topic 815
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4D

Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 4C
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4C

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- Definition

The portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments representing (a) the amount of the hedge ineffectiveness and (b) the amount, if any, excluded from the assessment of hedge effectiveness.

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-URI https://asc.fasb.org/1943274/2147480434/815-10-50-4C

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