Fair Value Measurements (Details) - USD ($) $ in Millions	Mar. 31, 2017	Mar. 31, 2016
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	$ 115	$ 94
Assets, Netting Adjustment	(46)	(31)
Assets, Total Net	69	63
Liabilities	(164)	(92)
Liabilities, Netting Adjustment	46	31
Liabilities, Total Net	(118)	(61)
Level 2 Instruments [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	115	93
Liabilities	(155)	(83)
Level 2 Instruments [Member] | Aluminium Contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	58	34
Liabilities	(138)	(28)
Level 2 Instruments [Member] | Currency Exchange Contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	56	59
Liabilities	(17)	(49)
Level 2 Instruments [Member] | Energy Contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	1	0
Liabilities	0	(5)
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	0	(1)
Level 3 Instruments [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	0	1
Liabilities	(9)	(9)
Level 3 Instruments [Member] | Energy Contracts [Member]
Derivative assets and liabilities measured and recognized at fair value on recurring basis
Assets	0	1
Liabilities	$ (9)	$ (9)