Description of Business and Accounting Policies - Accounting Pronouncements Recently Adopted (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of a change in accounting principle related to stock-based compensation		$ 687
Net cash provided by (used in) operating activities	$ 18,434	17,272	$ 12,039
Net cash provided by (used in) financing activities	9,860	(3,740)	(3,882)
Accounting Standards Update 2016-09
New Accounting Pronouncements or Change in Accounting Principle [Line Items]
Cumulative effect of a change in accounting principle related to stock-based compensation	$ 687
Net cash provided by (used in) operating activities		829	119
Net cash provided by (used in) financing activities		$ (829)	$ (119)