Income Taxes - Schedule of Changes in Valuation Allowance (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Income Tax Disclosure [Abstract]
Balance at beginning of year		$ 1,940	$ 1,291	$ 1,151
Increase to allowance	$ 1,267	831	772	180
Release of allowance		(246)	(209)	(42)
Acquisitions and divestitures (F)		(1)	(1)	29
Tax apportionment, tax rate and tax law changes		(24)	106	(15)
Foreign currency translation		84	(19)	(12)
Balance at end of year	$ 1,940	$ 2,584	$ 1,940	$ 1,291