Noncontrolling Interests (Details) - USD ($) $ / shares in Units, $ in Millions	3 Months Ended				6 Months Ended
	Jun. 30, 2019	Mar. 31, 2019	Jun. 30, 2018	Mar. 31, 2018	Jun. 30, 2019	Jun. 30, 2018
Noncontrolling Interests:
Minority Interest Ownership Percentage Threshold	100.00%				100.00%
Net income attributable to noncontrolling interest	$ 72		$ 66		$ 136	$ 121
Distributions to noncontrolling interest	39	$ 39	37	$ 39
Purchase of noncontrolling interest, net of tax	(148)	(89)	(192)	(118)
Change in noncontrolling interest ownership, net of tax	$ (6)	(7)	(5)	(6)
Common Noncontrolling Interest [Member]
Noncontrolling Interests:
Ownership percentage held by noncontrolling interest (percentage)	8.00%				8.00%
Net income attributable to noncontrolling interest	$ 34		$ 28		$ 60	$ 45
Purchase of noncontrolling interest (shares)	447,793		681,553		750,735	1,051,028
Purchase of noncontrolling interest (price per share)	$ 358.21		$ 295.31		$ 338.12	$ 312.38
Purchase price of noncontrolling interest	$ 161		$ 201		$ 254	$ 328
Preferred Noncontrolling Interest [Member]
Noncontrolling Interests:
Distributions to noncontrolling interest	37		37		75	75
Noncontrolling Interest [Member]
Noncontrolling Interests:
Distributions to noncontrolling interest	39	39	37	39
Purchase of noncontrolling interest, net of tax	(111)	(74)	(164)	(90)
Change in noncontrolling interest ownership, net of tax	(23)	(29)	(23)	(20)	(52)	(43)
Noncontrolling Interest [Member] | Common Noncontrolling Interest [Member]
Noncontrolling Interests:
Purchase of noncontrolling interest, net of tax	(111)		(164)		(185)	(254)
Additional Paid-in Capital [Member]
Noncontrolling Interests:
Distributions to noncontrolling interest	0	0	0	0
Purchase of noncontrolling interest, net of tax	(37)	(15)	(28)	(28)
Change in noncontrolling interest ownership, net of tax	17	$ 22	18	$ 14	39	32
Additional Paid-in Capital [Member] | Common Noncontrolling Interest [Member]
Noncontrolling Interests:
Impact of noncontrolling interest purchased	$ (37)		$ (28)		$ (52)	$ (56)