Financial Instruments and Commodity Contracts (Impact on AOCI and Earnings) (Details) - USD ($) $ in Millions	12 Months Ended
Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	$ (96)	$ 69	$ (72)
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	(26)	17	22
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	(96)	71	(83)
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	(26)	17	22
Cash Flow Hedging [Member] \| Aluminium Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	(137)	84	(26)
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	(27)	17	24
Cash Flow Hedging [Member] \| Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	48	(7)	(44)
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	2	1	(2)
Cash Flow Hedging [Member] \| Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	(7)	(5)	(12)
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	(1)	(1)	0
Cash Flow Hedging [Member] \| Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	0	(1)	(1)
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	0	0	0
Net Investment Hedging [Member] \| Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	0	(2)	11
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	$ 0	$ 0	$ 0

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.

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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.

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 10
-Section 50
-Paragraph 4C
-Subparagraph (d)
-URI http://asc.fasb.org/extlink&oid=56946850&loc=SL5624171-113959

Reference 2: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 815
-SubTopic 25
-Section 50
-Paragraph 1
-Subparagraph (a)(1),(2)
-URI http://asc.fasb.org/extlink&oid=6886632&loc=d3e76258-113986

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- Definition

The effective portion of gains and losses (net) on derivative instruments designated and qualifying as hedging instruments that was recognized in other comprehensive income during the current period.

+ References

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