Segment Information - Reconciliation (Details) - USD ($) $ in Millions	3 Months Ended			9 Months Ended
	Sep. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Segment Reporting Information [Line Items]
Total segment adjusted EBITDA	$ 2,223		$ 2,228	$ 6,666	$ 6,413
Depreciation and amortization	(1,235)		(1,285)	(3,619)	(3,858)
Impairment of goodwill	0	$ (6,500)	0	(6,506)	0
Stock-based compensation	(38)		(49)	(114)	(144)
Total other expense, net	(540)		(565)	(1,542)	(1,609)
INCOME (LOSS) BEFORE INCOME TAXES	410		329	(5,115)	802
Income tax expense	(108)		(57)	(377)	(123)
NET INCOME (LOSS)	302		272	(5,492)	679
Operating segments
Segment Reporting Information [Line Items]
Total segment adjusted EBITDA	3,980		4,172	12,005	12,535
Segment Reconciling Items
Segment Reporting Information [Line Items]
Total segment adjusted EBITDA	(1,757)		(1,944)	(5,339)	(6,122)
Depreciation and amortization	(1,235)		(1,285)	(3,619)	(3,858)
Impairment of goodwill	0		0	(6,506)	0
Other operating expenses	(1,757)		(1,944)	(5,339)	(6,122)
Operating income (loss)	950		894	(3,573)	2,411
Total other expense, net	$ (540)		$ (565)	$ (1,542)	$ (1,609)