Basis of Presentation and Significant Accounting Policies - Impact of Lease Standard Adoption (Details) - USD ($) $ in Millions	Jun. 30, 2019	[1]	Jan. 01, 2019	Dec. 31, 2018	[1]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets	$ 2,535		$ 2,460	$ 2,461
Noncurrent deferred tax assets and other noncurrent tax assets	1,935		1,913	1,924
Other noncurrent assets	4,388		4,149	2,799
Other current liabilities	10,110		11,011	10,753
Other noncurrent liabilities	6,183		6,910	5,850
Retained earnings	$ 94,440		89,574	$ 89,554
Accounting Standards Update 2016-02 [Member]
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Other current assets			(1)
Noncurrent deferred tax assets and other noncurrent tax assets			(11)
Other noncurrent assets			1,351
Other current liabilities			258
Other noncurrent liabilities			1,060
Retained earnings			$ 20

[1] Amounts may not add due to rounding.