Financial Instruments and Commodity Contracts (Gain (Loss) Recognition) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014	Mar. 31, 2013
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized	$ (39)	$ 3	$ 28
Other Expense (Income), Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Balance sheet remeasurement currency exchange contract losses	(13)	(19)	(6)
Realized (losses) gains, net	(2)	62	28
Unrealized (losses) gains on other derivative instruments, net	0	(10)	14
Total gain (loss) recognized	(15)	33	36
Other Expense (Income), Net [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized	(34)	(5)	8
Other Expense (Income), Net [Member] | Not Designated as Hedging Instrument [Member] | Aluminium Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized	(31)	(4)	(10)
Other Expense (Income), Net [Member] | Not Designated as Hedging Instrument [Member] | Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized	(5)	(15)	3
Other Expense (Income), Net [Member] | Not Designated as Hedging Instrument [Member] | Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized	2	14	15
Other Expense (Income), Net [Member] | Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized	$ 19	$ 38	$ 28