Goodwill and Other Intangible Assets - Changes in the Carrying Amount of Goodwill by Reporting Segment (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2019		Mar. 31, 2019	Sep. 30, 2019		Dec. 31, 2018
Goodwill And Intangible Assets Disclosure [Abstract]
Cumulative impairment charges	$ 1,370	[1]	$ 0	$ 1,370	[1]	$ 2,200

[1] Represents impairment of goodwill associated with our NCSA and EARC reporting units, resulting from our interim impairment assessment during the third quarter of 2019. In addition, in the third quarter of 2019 we recorded $143 million of impairment associated with NCSA intangible assets. The goodwill impairment values are included within the applicable operating segment’s operating loss. See Note 9, Goodwill and Other Intangible Assets, for further discussion.