Selected Quarterly Financial Data - Schedule of Selected Quarterly Financial Data (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended	12 Months Ended
Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Mar. 31, 2018	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Discrete income tax benefit for vested stock awards	$ 22			$ 56	$ 28	$ 64
Deferred tax expense (benefit) associated with changes in organizational entity structure			$ (90)			(81)
Pre-tax restructuring charge		$ 60				$ 60

Amount of expenses associated with exit or disposal activities pursuant to an authorized plan. Excludes expenses related to a discontinued operation or an asset retirement obligation.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
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Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef
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Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef
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-Name Accounting Standards Codification
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Period Type:	duration