Stockholders' Equity - Summary of Noncontrolling Interest Balances (Details) - USD ($) $ in Millions	3 Months Ended						9 Months Ended
	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Sep. 30, 2019	Sep. 30, 2018
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance			$ 1,112			$ 1,254	$ 1,112	$ 1,254
Acquisitions				$ 41		11		52
Sale of subsidiaries								32
Contributions from noncontrolling interests	$ 19		5				24
Distributions to noncontrolling interests	(12)	$ (10)	(11)	(9)	$ (11)	(9)	(33)	(29)
Net loss	(20)			(18)			(59)	(202)
Other comprehensive loss, net of tax							(4)	2
Ending balance	1,040			1,045			1,040	1,045
Tax reform, change in tax rate								150
Capital
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance			1,452			1,380	1,452	1,380
Acquisitions								52
Sale of subsidiaries								(37)
Contributions from noncontrolling interests							24
Distributions to noncontrolling interests							(33)	(29)
Net loss							0	0
Other comprehensive loss, net of tax							0	0
Ending balance	1,443			1,366			1,443	1,366
Accumulated Loss
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance			(332)			(126)	(332)	(126)
Acquisitions								0
Sale of subsidiaries								5
Contributions from noncontrolling interests							0
Distributions to noncontrolling interests							0	0
Net loss							(59)	(202)
Other comprehensive loss, net of tax							0	0
Ending balance	(391)			(323)			(391)	(323)
Accumulated Other Comprehensive Income (Loss)
Stockholders' Equity Attributable to Noncontrolling Interest [Roll Forward]
Beginning balance			$ (8)			$ 0	(8)	0
Acquisitions								0
Sale of subsidiaries								0
Contributions from noncontrolling interests							0
Distributions to noncontrolling interests							0	0
Net loss							0	0
Other comprehensive loss, net of tax							(4)	2
Ending balance	$ (12)			$ 2			$ (12)	$ 2