Financial Instruments and Commodity Contracts (Gain (Loss) Reclassification) (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]
(Loss) income before income taxes									$ 197	$ 8	$ 162
Income tax provision (benefit)	$ (41)	$ (47)	$ (27)	$ (36)	$ (18)	$ (16)	$ 3	$ (15)	(151)	(46)	(14)
Net income (loss)	$ 47	$ 64	$ (89)	$ 24	$ 29	$ 6	$ (13)	$ (60)	46	(38)	148
Cash Flow Hedging [Member]
Derivative Instruments, Gain (Loss) [Line Items]
(Loss) income before income taxes									(40)	8	(39)
Income tax provision (benefit)									12	(19)	8
Net income (loss)									(28)	(11)	(31)
Other expense (income), net [Member] | Cash Flow Hedging [Member] | Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									(4)	(5)	(5)
Other expense (income), net [Member] | Cash Flow Hedging [Member] | Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									0	(1)	(3)
Cost of goods sold [Member] | Cash Flow Hedging [Member] | Energy Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									(4)	(10)	0
Cost of goods sold [Member] | Cash Flow Hedging [Member] | Aluminium Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									(55)	83	(40)
Cost of goods sold [Member] | Cash Flow Hedging [Member] | Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									18	(44)	(14)
Net sales [Member] | Cash Flow Hedging [Member] | Aluminium Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									(3)	0	0
Net sales [Member] | Cash Flow Hedging [Member] | Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									7	(9)	18
Selling, general and administrative expenses [Member] | Cash Flow Hedging [Member] | Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									2	(4)	(1)
Gain on assets held-for-sale, net [Member] | Cash Flow Hedging [Member] | Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									0	0	7
Depreciation and amortization [Member] | Cash Flow Hedging [Member] | Currency Exchange Contracts [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									(1)	(1)	(1)
Interest expense [Member] | Cash Flow Hedging [Member] | Interest Rate Swap [Member]
Derivative Instruments, Gain (Loss) [Line Items]
Amount of Gain (Loss) Reclassified from AOCI into Income(Expense), Effective Portion									$ 0	$ (1)	$ 0