Financial Instruments and Commodity Contracts (Notional Amount (in kt)) (Details) - Mg	Mar. 31, 2025	Mar. 31, 2024
Aluminum Forward Sales Contracts \| Derivatives designated as hedging instruments: \| Cash flow sales
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	(847,000,000)	(755,000,000)
Aluminium Contracts [Member]
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	(982,000,000)	(1,061,000,000)
Aluminium Contracts [Member] \| Derivatives not designated as hedging instruments:
Derivative [Line Items]
Derivative, Nonmonetary Notional Amount	(135,000,000)	(306,000,000)

Nominal number of units used to calculate payment on derivative.

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 815
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 5
-Subparagraph (f)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480237/815-40-50-5

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 470
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1I
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481139/470-20-50-1I

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 505
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 18
-Subparagraph (c)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147481112/505-10-50-18

Reference 4: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1B
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1B

Reference 5: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 815
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1A
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480434/815-10-50-1A

Name:	us-gaap_DerivativeNonmonetaryNotionalAmount
Namespace Prefix:	us-gaap_
Data Type:	xbrli:decimalItemType
Balance Type:	na
Period Type:	instant