Financial Instruments and Commodity Contracts (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Mar. 31, 2014
Assets
Derivative Assets, Current	$ 67		$ 51
Derivative Assets, Noncurrent	9		4
Liabilities
Derivative Liabilities, Current	(65)		(60)
Derivative Liabilities, Noncurrent	(15)	[1]	(19)	[1]
Net Fair Value Assets/Liabilities	(4)		(24)
Designated as hedging instrument [Member]
Assets
Derivative Assets, Current	25		22
Derivative Assets, Noncurrent	9		4
Liabilities
Derivative Liabilities, Current	(21)		(22)
Derivative Liabilities, Noncurrent	(4)	[1]	(6)	[1]
Net Fair Value Assets/Liabilities	9		(2)
Designated as hedging instrument [Member] | Cash flow hedges [Member] | Aluminium contracts [Member]
Assets
Derivative Assets, Current	0		4
Derivative Assets, Noncurrent	0		0
Liabilities
Derivative Liabilities, Current	(13)		(7)
Derivative Liabilities, Noncurrent	0	[1]	0	[1]
Net Fair Value Assets/Liabilities	(13)		(3)
Designated as hedging instrument [Member] | Cash flow hedges [Member] | Currency exchange contracts [Member]
Assets
Derivative Assets, Current	23		15
Derivative Assets, Noncurrent	9		4
Liabilities
Derivative Liabilities, Current	(7)		(13)
Derivative Liabilities, Noncurrent	(4)	[1]	(6)	[1]
Net Fair Value Assets/Liabilities	21		0
Designated as hedging instrument [Member] | Cash flow hedges [Member] | Energy contracts [Member]
Assets
Derivative Assets, Current	2		3
Derivative Assets, Noncurrent	0		0
Liabilities
Derivative Liabilities, Current	0		0
Derivative Liabilities, Noncurrent	0	[1]	0	[1]
Net Fair Value Assets/Liabilities	2		3
Designated as hedging instrument [Member] | Cash flow hedges [Member] | Interest rate swaps [Member]
Assets
Derivative Assets, Current	0
Derivative Assets, Noncurrent	0
Liabilities
Derivative Liabilities, Current	(1)
Derivative Liabilities, Noncurrent	0	[1]
Net Fair Value Assets/Liabilities	(1)
Designated as hedging instrument [Member] | Net investment hedges [Member] | Currency exchange contracts [Member]
Assets
Derivative Assets, Current			0
Derivative Assets, Noncurrent			0
Liabilities
Derivative Liabilities, Current			(1)
Derivative Liabilities, Noncurrent			0	[1]
Net Fair Value Assets/Liabilities			(1)
Designated as hedging instrument [Member] | Fair value hedging [Member] | Aluminium contracts [Member]
Assets
Derivative Assets, Current			0
Derivative Assets, Noncurrent			0
Liabilities
Derivative Liabilities, Current			(1)
Derivative Liabilities, Noncurrent			0	[1]
Net Fair Value Assets/Liabilities			(1)
Not designated as hedging instrument [Member]
Assets
Derivative Assets, Current	42		29
Derivative Assets, Noncurrent	0		0
Liabilities
Derivative Liabilities, Current	(44)		(38)
Derivative Liabilities, Noncurrent	(11)	[1]	(13)	[1]
Net Fair Value Assets/Liabilities	(13)		(22)
Not designated as hedging instrument [Member] | Aluminium contracts [Member]
Assets
Derivative Assets, Current	16		19
Derivative Assets, Noncurrent	0		0
Liabilities
Derivative Liabilities, Current	(29)		(28)
Derivative Liabilities, Noncurrent	0	[1]	0	[1]
Net Fair Value Assets/Liabilities	(13)		(9)
Not designated as hedging instrument [Member] | Currency exchange contracts [Member]
Assets
Derivative Assets, Current	26		9
Derivative Assets, Noncurrent	0		0
Liabilities
Derivative Liabilities, Current	(9)		(3)
Derivative Liabilities, Noncurrent	0	[1]	0	[1]
Net Fair Value Assets/Liabilities	17		6
Not designated as hedging instrument [Member] | Energy contracts [Member]
Assets
Derivative Assets, Current	0		1
Derivative Assets, Noncurrent	0		0
Liabilities
Derivative Liabilities, Current	(6)		(7)
Derivative Liabilities, Noncurrent	(11)	[1]	(13)	[1]
Net Fair Value Assets/Liabilities	$ (17)		$ (19)

[1] The noncurrent portions of derivative assets and liabilities are included in “Other long-term assets-third parties” and in “Other long-term liabilities” respectively, in the accompanying condensed consolidated balance sheets