Quarterly Consolidated Financial Information - Schedule of Quarterly Consolidated Financial Information (Parenthetical) (Detail) - USD ($) $ in Millions	3 Months Ended
	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017
Quarterly Financial Information Disclosure [Abstract]
Losses (gains) on sales of facilities	$ 6	$ 5	$ 8	$ 305	$ 1	$ 4	$ 1	$ 1
Losses on retirement of debt		$ 7				$ 25