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