Financial Instruments and Commodity Contracts (Details Textual) In Millions, unless otherwise specified	12 Months Ended						12 Months Ended									12 Months Ended									12 Months Ended				12 Months Ended		12 Months Ended
	Mar. 31, 2015 USD ($)	Mar. 31, 2014 USD ($)	Mar. 31, 2015 Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2014 Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2015 Not Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2014 Not Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2015 Aluminum Forward Sales Contracts [Member] Not Designated as Hedging Instrument [Member]	Mar. 31, 2015 Aluminium Contracts [Member] Mg	Mar. 31, 2014 Aluminium Contracts [Member] Mg	Mar. 31, 2015 Aluminium Contracts [Member] Not Designated as Hedging Instrument [Member] USD ($) Mg	Mar. 31, 2014 Aluminium Contracts [Member] Not Designated as Hedging Instrument [Member] USD ($) Mg	Mar. 31, 2015 Currency Exchange Contracts [Member] Not Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2014 Currency Exchange Contracts [Member] Not Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2015 Electricity Swaps [Member] Not Designated as Hedging Instrument [Member] USD ($) MWh	Mar. 31, 2014 Electricity Swaps [Member] Not Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2015 Natural Gas Swaps [Member]	Mar. 31, 2015 Natural Gas Swaps [Member] Designated as Hedging Instrument [Member] USD ($) MMBTU	Mar. 31, 2014 Natural Gas Swaps [Member] Designated as Hedging Instrument [Member] USD ($) MMBTU	Mar. 31, 2015 Natural Gas Swaps [Member] Not Designated as Hedging Instrument [Member] USD ($) MMBTU	Mar. 31, 2014 Natural Gas Swaps [Member] Not Designated as Hedging Instrument [Member] USD ($) MMBTU	Mar. 31, 2015 Interest Rate Swaps [Member] Designated as Hedging Instrument [Member] Loans due December 2014 through 2015 [Member] USD ($)	Mar. 31, 2015 Interest Rate Swaps [Member] Designated as Hedging Instrument [Member] Loans due December 2014 through 2015 [Member] KRW	Mar. 31, 2014 Interest Rate Swaps [Member] Designated as Hedging Instrument [Member] Loans due December 2014 through 2015 [Member] USD ($)	Mar. 31, 2014 Interest Rate Swaps [Member] Designated as Hedging Instrument [Member] Loans due December 2014 through 2015 [Member] KRW	Mar. 31, 2015 Fair Value Hedging [Member] Aluminum Forward Purchase Contracts [Member] Designated as Hedging Instrument [Member] Mg	Mar. 31, 2014 Fair Value Hedging [Member] Aluminum Forward Purchase Contracts [Member] Designated as Hedging Instrument [Member] Mg	Mar. 31, 2015 Fair Value Hedging [Member] Aluminium Contracts [Member] Designated as Hedging Instrument [Member] Mg	Mar. 31, 2014 Fair Value Hedging [Member] Aluminium Contracts [Member] Designated as Hedging Instrument [Member] USD ($) Mg	Mar. 31, 2015 Cash Flow Hedges [Member] Aluminum Forward Purchase Contracts [Member] Designated as Hedging Instrument [Member] Mg	Mar. 31, 2014 Cash Flow Hedges [Member] Aluminum Forward Purchase Contracts [Member] Designated as Hedging Instrument [Member] Mg	Mar. 31, 2015 Cash Flow Hedges [Member] Aluminum Forward Sales Contracts [Member] Designated as Hedging Instrument [Member] Mg	Mar. 31, 2014 Cash Flow Hedges [Member] Aluminum Forward Sales Contracts [Member] Designated as Hedging Instrument [Member] Mg	Mar. 31, 2015 Cash Flow Hedges [Member] Aluminium Contracts [Member] Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2014 Cash Flow Hedges [Member] Aluminium Contracts [Member] Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2015 Cash Flow Hedges [Member] Currency Exchange Contracts [Member] USD ($)	Mar. 31, 2014 Cash Flow Hedges [Member] Currency Exchange Contracts [Member] USD ($)	Mar. 31, 2015 Cash Flow Hedges [Member] Currency Exchange Contracts [Member] Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2014 Cash Flow Hedges [Member] Currency Exchange Contracts [Member] Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2015 Cash Flow Hedges [Member] Interest Rate Swaps [Member] Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2015 Net Investment Hedging [Member] Currency Exchange Contracts [Member] USD ($)	Mar. 31, 2014 Net Investment Hedging [Member] Currency Exchange Contracts [Member] USD ($)	Mar. 31, 2015 Net Investment Hedging [Member] Currency Exchange Contracts [Member] Designated as Hedging Instrument [Member] USD ($)	Mar. 31, 2014 Net Investment Hedging [Member] Currency Exchange Contracts [Member] Designated as Hedging Instrument [Member] USD ($)
Financial Instruments And Commodity Contracts [Abstract]
Higher remaining maturity range							0 years 6 months									1 year 0 months									2 years 0 months				2 years 0 months		2 years 0 months
Notional amount (in tons)								(318,000)	(302,000)	(36,000)	(105,000)						7,000,000	9,500,000	2,000,000	1,500,000					2,000	9,000	2,000	9,000	1,000	16,000	(285,000)	(222,000)
Notional amount												$ 868	$ 649								$ 78	86,000	$ 127	136,000											$ 590	$ 724				$ 28	$ 61
Notional outstanding (less than)														1,000,000
Derivative asset (liability) (less than)	(96)	(24)	(47)	(2)	(49)	(22)				(2)	(9)	(28)	6	16	(19)		8	3	(3)	(1)								(1)					10	(3)			(53)	0	(1)			5	(1)
Interest rate swaps, hedged amount																					78	86,000
Derivative, fixed interest rate																					3.69%	3.69%
Expected reclassification of gains (losses) from AOCI to earnings	$ (36)