New Accounting Standards (Details) - USD ($) $ in Millions		3 Months Ended		9 Months Ended
		Sep. 30, 2018	Sep. 30, 2017	Sep. 30, 2018	Sep. 30, 2017
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating expenses	[1]	$ (4,985)	$ (4,849)	$ (14,899)	$ (14,021)
Selling, general and administrative expenses	[1]	(1,018)	$ (1,107)	(2,758)	(2,848)
Pro Forma | Minimum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset		3,000		3,000
Operating lease, liability		3,000		3,000
Pro Forma | Maximum
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating lease, right-of-use asset		4,000		4,000
Operating lease, liability		$ 4,000		$ 4,000
Accounting Standards Update 2017-07
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Operating expenses					234
Selling, general and administrative expenses					$ 251

[1] 2017 adjusted to conform to ASU 2017-07. Refer to Note 5, "New Accounting Standards" beginning on page 10.