Financial Instruments and Commodity Contracts (Impact on AOCI and Earnings) (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]
Amount of gain (loss) recognized in OCI (effective portion)	$ (72)	$ 17	$ 14
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	22	40	31
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	(83)	20	13
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	22	40	31
Cash Flow Hedging [Member] | Aluminium Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	(26)	35	34
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	24	39	29
Cash Flow Hedging [Member] | Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	(44)	(16)	(21)
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	(2)	1	2
Cash Flow Hedging [Member] | Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	(12)	1	1
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	0	0	0
Cash Flow Hedging [Member] | Interest Rate Swaps [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	(1)	0	(1)
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	0	0	0
Net Investment Hedging [Member] | Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of gain (loss) recognized in OCI (effective portion)	11	(3)	1
Amount of gain (loss) recognized in other (income) expense, net (ineffective and excluded portion)	$ 0	$ 0	$ 0