Fair Value Measurements (Details) (USD $) In Millions, unless otherwise specified	Mar. 31, 2015	Mar. 31, 2014
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	$ 77	$ 55
Assets, Netting Adjustment	(28)	(20)
Assets, Total Net	49	35
Liabilities	(173)	(79)
Liabilities, Netting Adjustment	28	20
Liabilities, Total Net	(145)	(59)
Level 2 Instruments [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	77	55
Liabilities	(157)	(60)
Level 2 Instruments [Member] | Aluminium Contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	39	23
Liabilities	(31)	(36)
Level 2 Instruments [Member] | Currency Exchange Contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	35	28
Liabilities	(111)	(23)
Level 2 Instruments [Member] | Energy Contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	3	4
Liabilities	(14)	(1)
Level 2 Instruments [Member] | Interest Rate Swaps [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	(16)	(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	$ (16)	$ (19)