Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Jun. 30, 2014		Mar. 31, 2014
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	$ 76		$ 55
Liabilities	(80)		(79)
Derivative Asset, Master Netting Adjustment	(21)	[1]	(20)	[1]
Derivative Liability, Master Netting Adjustment	21	[1]	20	[1]
Derivative Asset	55		35
Derivative Liability	59		59
Level 2 Instruments [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	76		55
Liabilities	(63)		(60)
Level 2 Instruments [Member] | Aluminium contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	16		23
Liabilities	(42)		(36)
Level 2 Instruments [Member] | Currency exchange contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	58		28
Liabilities	(20)		(23)
Level 2 Instruments [Member] | Energy contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	2		4
Liabilities	0		(1)
Level 2 Instruments [Member] | Interest Rate Swap [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	0		0
Liabilities	(1)		0
Level 3 Instruments [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	0		0
Liabilities	(17)		(19)
Level 3 Instruments [Member] | Energy contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	0		0
Liabilities	$ (17)		$ (19)

[1] (A) Amounts represent the impact of legally enforceable master netting agreements that allow the Company to settle positive and negative positions with the same counterparties.