Quarterly Data (Details) - USD ($)	1 Months Ended	3 Months Ended									12 Months Ended
	Dec. 31, 2017	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Sales		$ 3,472,000,000	$ 3,524,000,000	$ 3,573,000,000	$ 3,445,000,000	$ 3,271,000,000	$ 3,236,000,000	$ 3,261,000,000	$ 3,192,000,000		$ 14,014,000,000	$ 12,960,000,000	$ 12,394,000,000
Net income		$ 218,000,000	$ 161,000,000	$ 120,000,000	$ 143,000,000	$ (727,000,000)	$ 119,000,000	$ 212,000,000	$ 322,000,000		$ 642,000,000	$ (74,000,000)	$ (941,000,000)
Basic
Net loss per share-basic (in usd per share)		$ 0.45	$ 0.33	$ 0.25	$ 0.30	$ (1.51)	$ 0.23	$ 0.44	$ 0.69		$ 1.33	$ (0.28)	$ (2.31)
Diluted
Net loss per share-diluted (in usd per share)		$ 0.44	$ 0.32	$ 0.24	$ 0.29	$ (1.51)	$ 0.22	$ 0.43	$ 0.65		$ 1.30	$ (0.28)	$ (2.31)
Gain on disposition of assets, after tax											$ 81,000,000
Gain (loss) on disposition of business											109,000,000	$ (57,000,000)	$ (3,000,000)
Payment for pension benefits, net of tax		$ 72,000,000
Payment for pension benefits		92,000,000									298,000,000	310,000,000	290,000,000
Impairment of goodwill (A and O)					$ 0						0	719,000,000	0
Tax cuts and jobs act of 2017, incomplete accounting, provisional income tax expense (benefit)	$ 272,000,000					$ 272,000,000					59,000,000
Favorable adjustment to a separation-related guarantee liability (after-tax)												16,000,000
Favorable adjustment to a separation-related guarantee liability (pre-tax)												25,000,000
Firth Rixson
Diluted
Business combination, contingent consideration arrangements, change in amount of contingent consideration, liability, net of tax												81,000,000
Contingent consideration arrangements, change in amount of contingent consideration, liability						(81,000,000)				$ (56,000,000)		81,000,000	56,000,000
Engineered Products and Solutions
Segment Reporting Information [Line Items]
Sales											6,316,000,000	5,943,000,000	$ 5,728,000,000
Diluted
Goodwill, impairment loss, net of tax						719,000,000
Impairment of goodwill (A and O)						$ 719,000,000						$ 719,000,000
Texarkana, Texas Rolling Mill
Diluted
Gain on disposition of assets, after tax		119,000,000
Gain (loss) on disposition of business		154,000,000
Eger
Diluted
Gain on disposition of assets, after tax		(39,000,000)
Gain (loss) on disposition of business		$ (43,000,000)									$ (43,000,000)