Cash, Cash Equivalents, and Marketable Securities - Reconciliation to Cash Flow (Details) - USD ($) $ in Millions	Sep. 30, 2018	Jun. 30, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Dec. 31, 2016	Sep. 30, 2016
Reconciliation to Cash Flow [Abstract]
Cash and cash equivalents	$ 20,425		$ 20,522
Restricted cash included in accounts receivable, net and other	604		1,329
Restricted cash included in other assets	3		5
Total cash, cash equivalents, and restricted cash shown in the consolidated statements of cash flows	$ 21,032	$ 20,536	$ 21,856	$ 13,960	$ 13,851	$ 19,934	$ 14,124