Asset Impairments and Unusual Items - Components (Detail) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Asset Impairments and Unusual Items
(Income) expense from divestitures	$ (38)	$ 9	$ (7)
Asset impairments	41	59	89
Other	(19)	44
(Income) expense from divestitures, asset impairments and unusual items	$ (16)	$ 112	$ 82