Acquisitions and Divestitures - 2014 Acquisitions (Details) - Firth Rixson - USD ($)	3 Months Ended		12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2018	Nov. 30, 2014
Business Acquisition [Line Items]
Contingent consideration liability	$ 0		$ 0		$ 0
Contingent consideration arrangements, change in amount of contingent consideration, liability	$ 81,000,000	$ 56,000,000	$ (81,000,000)	$ (56,000,000)
Maximum
Business Acquisition [Line Items]
Contingent consideration liability						$ 150,000,000