Financial Instruments and Commodity Contracts (Gain (Loss) Recognized on Fair Value Hedges of Metal Price Risk) (Details) - Designated as Hedging Instrument [Member] - Fair Value Hedging [Member] - USD ($) $ in Millions	12 Months Ended
	Mar. 31, 2017	Mar. 31, 2016
Forward Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Derivative Contracts	$ 0	$ (2)
Designated Hedged Items	0	2
Aluminum Forward Purchase Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Net Ineffectiveness	$ 0	$ 0