Financial Instruments and Commodity Contracts (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Assets
Derivative Assets, Current	$ 77us-gaap_DerivativeAssetsCurrent	$ 51us-gaap_DerivativeAssetsCurrent
Derivative Asset, Noncurrent	0us-gaap_DerivativeAssetsNoncurrent	4us-gaap_DerivativeAssetsNoncurrent
Liabilities
Derivative Liabilities, Current	(149)us-gaap_DerivativeLiabilitiesCurrent	(60)us-gaap_DerivativeLiabilitiesCurrent
Derivative Liabilities, Noncurrent	(24)us-gaap_DerivativeLiabilitiesNoncurrent	(19)us-gaap_DerivativeLiabilitiesNoncurrent
Derivative Assets (Liabilities), at Fair Value, Net	(96)	(24)
Designated as Hedging Instrument [Member]
Assets
Derivative Assets, Current	24	22
Derivative Asset, Noncurrent	0	4
Liabilities
Derivative Liabilities, Current	(54)	(22)
Derivative Liabilities, Noncurrent	(17)	(6)
Derivative Assets (Liabilities), at Fair Value, Net	(47)	(2)
Designated as Hedging Instrument [Member] \| Cash Flow Hedges [Member] \| Aluminium Contracts [Member]
Assets
Derivative Assets, Current	15	4
Derivative Asset, Noncurrent	0	0
Liabilities
Derivative Liabilities, Current	(5)	(7)
Derivative Liabilities, Noncurrent	0	0
Derivative Assets (Liabilities), at Fair Value, Net	10	(3)
Designated as Hedging Instrument [Member] \| Cash Flow Hedges [Member] \| Currency Exchange Contracts [Member]
Assets
Derivative Assets, Current	4	15
Derivative Asset, Noncurrent	0	4
Liabilities
Derivative Liabilities, Current	(42)	(13)
Derivative Liabilities, Noncurrent	(15)	(6)
Derivative Assets (Liabilities), at Fair Value, Net	(53)	0
Designated as Hedging Instrument [Member] \| Cash Flow Hedges [Member] \| Energy Contracts [Member]
Assets
Derivative Assets, Current	0	3
Derivative Asset, Noncurrent	0	0
Liabilities
Derivative Liabilities, Current	(6)	0
Derivative Liabilities, Noncurrent	(2)	0
Derivative Assets (Liabilities), at Fair Value, Net	(8)	3
Designated as Hedging Instrument [Member] \| Cash Flow Hedges [Member] \| Interest Rate Swaps [Member]
Assets
Derivative Assets, Current	0
Derivative Asset, Noncurrent	0
Liabilities
Derivative Liabilities, Current	(1)
Derivative Liabilities, Noncurrent	0
Derivative Assets (Liabilities), at Fair Value, Net	(1)
Designated as Hedging Instrument [Member] \| Net Investment Hedges [Member] \| Currency Exchange Contracts [Member]
Assets
Derivative Assets, Current	5	0
Derivative Asset, Noncurrent	0	0
Liabilities
Derivative Liabilities, Current	0	(1)
Derivative Liabilities, Noncurrent	0	0
Derivative Assets (Liabilities), at Fair Value, Net	5	(1)
Designated as Hedging Instrument [Member] \| Fair Value Hedges [Member] \| Aluminium Contracts [Member]
Assets
Derivative Assets, Current		0
Derivative Asset, Noncurrent		0
Liabilities
Derivative Liabilities, Current		(1)
Derivative Liabilities, Noncurrent		0
Derivative Assets (Liabilities), at Fair Value, Net		(1)
Not Designated as Hedging Instrument [Member]
Assets
Derivative Assets, Current	53	29
Derivative Asset, Noncurrent	0	0
Liabilities
Derivative Liabilities, Current	(95)	(38)
Derivative Liabilities, Noncurrent	(7)	(13)
Derivative Assets (Liabilities), at Fair Value, Net	(49)	(22)
Not Designated as Hedging Instrument [Member] \| Aluminium Contracts [Member]
Assets
Derivative Assets, Current	24	19
Derivative Asset, Noncurrent	0	0
Liabilities
Derivative Liabilities, Current	(26)	(28)
Derivative Liabilities, Noncurrent	0	0
Derivative Assets (Liabilities), at Fair Value, Net	(2)	(9)
Not Designated as Hedging Instrument [Member] \| Currency Exchange Contracts [Member]
Assets
Derivative Assets, Current	26	9
Derivative Asset, Noncurrent	0	0
Liabilities
Derivative Liabilities, Current	(54)	(3)
Derivative Liabilities, Noncurrent	0	0
Derivative Assets (Liabilities), at Fair Value, Net	(28)	6
Not Designated as Hedging Instrument [Member] \| Energy Contracts [Member]
Assets
Derivative Assets, Current	3	1
Derivative Asset, Noncurrent	0	0
Liabilities
Derivative Liabilities, Current	(15)	(7)
Derivative Liabilities, Noncurrent	(7)	(13)
Derivative Assets (Liabilities), at Fair Value, Net	$ (19)	$ (19)

Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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Fair value, after the effects of master netting arrangements, of a financial asset or other contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

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- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled within one year or normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References+ Details

- Definition

Fair value, after the effects of master netting arrangements, of a financial liability or contract with one or more underlyings, notional amount or payment provision or both, and the contract can be net settled by means outside the contract or delivery of an asset, expected to be settled after one year or the normal operating cycle, if longer. Includes assets not subject to a master netting arrangement and not elected to be offset.

+ References+ Details