Financial Instruments and Commodity Contracts (Notional Amount (in kt)) (Details) - Mg Mg in Thousands	Mar. 31, 2017	Mar. 31, 2016
Aluminum Forward Sales Contracts [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Notional amount (in tons)	(391)	(301)
Aluminium Contracts [Member]
Derivative [Line Items]
Notional amount (in tons)	(480)	(376)
Aluminium Contracts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount (in tons)	(89)	(76)