Financial Instruments and Commodity Contracts Financial Instruments and Commodity Contracts (Details 2)	Sep. 30, 2014 Mg	Mar. 31, 2014 Mg
Aluminium contracts [Member]
Derivative [Line Items]
Notional amount (in tons)	(419,000)	(302,000)
Designated as hedging instrument [Member] | Aluminum forward purchase contracts [Member] | Cash flow hedges [Member]
Derivative [Line Items]
Notional amount (in tons)	2,000	16,000
Designated as hedging instrument [Member] | Aluminum forward purchase contracts [Member] | Fair value hedging [Member]
Derivative [Line Items]
Notional amount (in tons)	6,000	9,000
Designated as hedging instrument [Member] | Aluminum foward sales contracts [Member] | Cash flow hedges [Member]
Derivative [Line Items]
Notional amount (in tons)	(307,000)	(222,000)
Designated as hedging instrument [Member] | Aluminium contracts [Member] | Fair value hedging [Member]
Derivative [Line Items]
Notional amount (in tons)	6,000	9,000
Not designated as hedging instrument [Member] | Aluminium contracts [Member]
Derivative [Line Items]
Notional amount (in tons)	(120,000)	(105,000)