Financial Instruments and Commodity Contracts (Gain (Loss) Recognized on Fair Value Hedges of Metal Price Risk) (Details) (Designated as Hedging Instrument [Member], Aluminum Forward Purchase Contracts [Member], Fair Value Hedging [Member], USD $) In Millions, unless otherwise specified	12 Months Ended
	Mar. 31, 2015	Mar. 31, 2014
Designated as Hedging Instrument [Member] | Aluminum Forward Purchase Contracts [Member] | Fair Value Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Contracts	$ 0	$ (3)
Designated Hedged Items	0	3
Net Ineffectiveness	$ 0	$ 0