Summary of Significant Accounting Policies - Recently Adopted Accounting Guidance (Details) - USD ($) shares in Thousands, $ in Millions		12 Months Ended
		Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Income tax expense (benefit)		$ 1,136	$ 1,250	$ 1,210
Distributions from partnership investments	[1]	144	137	111
Other, net (operating)	[2]	(10)	71	(30)
Net cash provided by operating activities	[1],[2]	4,026	2,933	2,158
Net cash provided by (used in) investing activities	[1]	(2,574)	(1,736)	(1,866)
Other, net (financing)	[2]	(72)	(97)	(47)
Net cash provided by (used in) financing activities	[2]	$ (225)	$ (609)	$ (1,635)
Common stock equivalents		4,287	4,443	4,714
Accounting Standards Update 2016-09 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net cash provided by (used in) financing activities		$ 72
Accounting Standards Update 2016-09 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Income tax expense (benefit)		$ (29)
Net cash provided by operating activities			$ (79)	$ (53)
Net cash provided by (used in) financing activities			79	53
Common stock equivalents		1,000
Accounting Standards Update 2016-15 [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net cash provided by operating activities		$ 144
Accounting Standards Update 2016-15 [Member] | Restatement Adjustment [Member]
New Accounting Pronouncements Or Change In Accounting Principle [Line Items]
Net cash provided by operating activities			137	111
Net cash provided by (used in) investing activities			$ (137)	$ (111)

[1] As required in adopting ASU 2016-15, the Company retrospectively reclassified $137 million in 2015 and $111 million in 2014 of cash distributions of earnings from partnership investments from investing to operating activities. The comparable amount reported in operating activities in 2016 was $144 million. See Note 2 for further discussion.

[2] As required in adopting Accounting Standard Update ("ASU") 2016-09, the Company retrospectively reclassified $79 million in 2015 and $53 million in 2014 of cash payments from operating to financing activities. These payments were related to employee tax obligations associated with stock compensation. The comparable amount reported in financing activities in 2016 was $72 million. See Note 2 for further discussion.