Financial Instruments and Commodity Contracts (Notional Amount (in kt)) (Details)	Mar. 31, 2015 Mg	Mar. 31, 2014 Mg
Aluminum Forward Purchase Contracts [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Notional amount (in tons)	1,000	16,000
Aluminum Forward Purchase Contracts [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative [Line Items]
Notional amount (in tons)	2,000	9,000
Aluminum Forward Sales Contracts [Member] | Designated as Hedging Instrument [Member] | Cash Flow Hedging [Member]
Derivative [Line Items]
Notional amount (in tons)	(285,000)	(222,000)
Aluminium Contracts [Member]
Derivative [Line Items]
Notional amount (in tons)	(318,000)	(302,000)
Aluminium Contracts [Member] | Designated as Hedging Instrument [Member] | Fair Value Hedging [Member]
Derivative [Line Items]
Notional amount (in tons)	2,000	9,000
Aluminium Contracts [Member] | Not Designated as Hedging Instrument [Member]
Derivative [Line Items]
Notional amount (in tons)	(36,000)	(105,000)