Financial Instruments and Commodity Contracts (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Assets
Derivative Assets, Current	$ 113	$ 88
Derivative Asset, Noncurrent	2	6
Liabilities
Derivative Liabilities, Current	(151)	(85)
Derivative Liabilities, Noncurrent	(13)	(7)
Derivative Assets (Liabilities), at Fair Value, Net	(49)	2
Designated as Hedging Instrument [Member]
Assets
Derivative Assets, Current	27	25
Derivative Asset, Noncurrent	1	5
Liabilities
Derivative Liabilities, Current	(70)	(10)
Derivative Liabilities, Noncurrent	(12)	(6)
Derivative Assets (Liabilities), at Fair Value, Net	(54)	14
Designated as Hedging Instrument [Member] \| Natural Gas Swaps [Member]
Liabilities
Derivative Assets (Liabilities), at Fair Value, Net	(1)	(4)
Designated as Hedging Instrument [Member] \| Cash Flow Hedges [Member] \| Aluminium Contracts [Member]
Assets
Derivative Assets, Current	0	10
Derivative Asset, Noncurrent	0	0
Liabilities
Derivative Liabilities, Current	(69)	(2)
Derivative Liabilities, Noncurrent	0	0
Derivative Assets (Liabilities), at Fair Value, Net	(69)	8
Designated as Hedging Instrument [Member] \| Cash Flow Hedges [Member] \| Currency Exchange Contracts [Member]
Assets
Derivative Assets, Current	26	15
Derivative Asset, Noncurrent	1	5
Liabilities
Derivative Liabilities, Current	(1)	(3)
Derivative Liabilities, Noncurrent	(3)	(5)
Derivative Assets (Liabilities), at Fair Value, Net	23	12
Designated as Hedging Instrument [Member] \| Cash Flow Hedges [Member] \| Energy Contracts [Member]
Assets
Derivative Assets, Current	1	0
Derivative Asset, Noncurrent	0	0
Liabilities
Derivative Liabilities, Current	0	(4)
Derivative Liabilities, Noncurrent	(9)	0
Derivative Assets (Liabilities), at Fair Value, Net	(8)	(4)
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		0
Derivative Liabilities, Noncurrent		(1)
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		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	86	63
Derivative Asset, Noncurrent	1	1
Liabilities
Derivative Liabilities, Current	(81)	(75)
Derivative Liabilities, Noncurrent	(1)	(1)
Derivative Assets (Liabilities), at Fair Value, Net	5	(12)
Not Designated as Hedging Instrument [Member] \| Natural Gas Swaps [Member]
Liabilities
Derivative Assets (Liabilities), at Fair Value, Net	(1)	(1)
Not Designated as Hedging Instrument [Member] \| Fuel [Member]
Liabilities
Derivative Assets (Liabilities), at Fair Value, Net	(1)
Not Designated as Hedging Instrument [Member] \| Aluminium Contracts [Member]
Assets
Derivative Assets, Current	57	24
Derivative Asset, Noncurrent	1	0
Liabilities
Derivative Liabilities, Current	(68)	(26)
Derivative Liabilities, Noncurrent	(1)	0
Derivative Assets (Liabilities), at Fair Value, Net	(11)	(2)
Not Designated as Hedging Instrument [Member] \| Currency Exchange Contracts [Member]
Assets
Derivative Assets, Current	29	39
Derivative Asset, Noncurrent	0	0
Liabilities
Derivative Liabilities, Current	(13)	(39)
Derivative Liabilities, Noncurrent	0	(1)
Derivative Assets (Liabilities), at Fair Value, Net	$ 16	(1)
Not Designated as Hedging Instrument [Member] \| Energy Contracts [Member]
Assets
Derivative Assets, Current		0
Derivative Asset, Noncurrent		1
Liabilities
Derivative Liabilities, Current		(10)
Derivative Liabilities, Noncurrent		0
Derivative Assets (Liabilities), at Fair Value, Net		$ (9)

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