Financial Instruments and Commodity Contracts (Gain (Loss) Recognition) (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2015	Sep. 30, 2015	Jun. 30, 2015	Mar. 31, 2017	Mar. 31, 2016	Mar. 31, 2015
Derivative Instruments, Gain (Loss) [Line Items]
Income tax provision	$ 41	$ 47	$ 27	$ 36	$ 18	$ 16	$ (3)	$ 15	$ 151	$ 46	$ 14
Total gain (loss) recognized									15	27	(39)
Other Expense (Income), Net [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Balance sheet remeasurement currency exchange contract gains (losses)									35	(53)	(13)
Realized gains (losses), net									(61)	64	(2)
Unrealized gains (losses) on other derivative instruments, net									5	(4)	0
Total gain (loss) recognized									(21)	7	(15)
Other Expense (Income), Net [Member] | Not Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized									4	(10)	(34)
Other Expense (Income), Net [Member] | Not Designated as Hedging Instrument [Member] | Aluminium Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized									(44)	47	(31)
Other Expense (Income), Net [Member] | Not Designated as Hedging Instrument [Member] | Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized									40	(60)	(5)
Other Expense (Income), Net [Member] | Not Designated as Hedging Instrument [Member] | Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized									8	3	2
Other Expense (Income), Net [Member] | Designated as Hedging Instrument [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Total gain (loss) recognized									(25)	17	19
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Income tax provision									$ (12)	$ 19	$ (8)