Cash, Cash Equivalents, and Marketable Securities - Fair Values on Recurring Basis (Details) - USD ($) $ in Millions	Jun. 30, 2018	Dec. 31, 2017
Total Estimated Fair Value
Cash equivalents and marketable securities	$ 19,516
Cost or Amortized Cost
Cash equivalents and marketable securities	19,567
Recurring
Schedule of Investments [Line Items]
Cash	8,107	$ 9,982
Total Estimated Fair Value
Cash, cash equivalents and short-term investments	27,766	32,321
Less: Restricted cash, cash equivalents, and marketable securities	(716)	(1,335)
Total cash, cash equivalents, and marketable securities	27,050	30,986
Cost or Amortized Cost
Cash, cash equivalents and short-term investments	27,728
Gross Unrealized Gains
Short-term investments	91
Gross Unrealized Losses
Short-term investments	(53)
Recurring | Level 1 securities
Total Estimated Fair Value
Equity securities	108	53
Cost or Amortized Cost
Equity securities	25
Gross Unrealized Gains
Equity securities	83
Gross Unrealized Losses
Equity securities	0
Recurring | Level 1 securities | Money market funds
Schedule of Investments [Line Items]
Money market funds	10,094	11,343
Recurring | Level 2 securities
Total Estimated Fair Value
Equity securities	35	0
Cost or Amortized Cost
Equity securities	29
Gross Unrealized Gains
Equity securities	6
Gross Unrealized Losses
Equity securities	0
Recurring | Level 2 securities | Foreign government and agency securities
Total Estimated Fair Value
Cash equivalents and marketable securities	389	620
Cost or Amortized Cost
Cash equivalents and marketable securities	389
Gross Unrealized Gains
Cash equivalents and marketable securities	0
Gross Unrealized Losses
Cash equivalents and marketable securities	0
Recurring | Level 2 securities | U.S. government and agency securities
Total Estimated Fair Value
Cash equivalents and marketable securities	5,441	4,823
Cost or Amortized Cost
Cash equivalents and marketable securities	5,465
Gross Unrealized Gains
Cash equivalents and marketable securities	1
Gross Unrealized Losses
Cash equivalents and marketable securities	(25)
Recurring | Level 2 securities | Corporate debt securities
Total Estimated Fair Value
Cash equivalents and marketable securities	2,621	4,257
Cost or Amortized Cost
Cash equivalents and marketable securities	2,638
Gross Unrealized Gains
Cash equivalents and marketable securities	1
Gross Unrealized Losses
Cash equivalents and marketable securities	(18)
Recurring | Level 2 securities | Asset-backed securities
Total Estimated Fair Value
Cash equivalents and marketable securities	748	905
Cost or Amortized Cost
Cash equivalents and marketable securities	756
Gross Unrealized Gains
Cash equivalents and marketable securities	0
Gross Unrealized Losses
Cash equivalents and marketable securities	(8)
Recurring | Level 2 securities | Other fixed income securities
Total Estimated Fair Value
Cash equivalents and marketable securities	223	$ 338
Cost or Amortized Cost
Cash equivalents and marketable securities	225
Gross Unrealized Gains
Cash equivalents and marketable securities	0
Gross Unrealized Losses
Cash equivalents and marketable securities	$ (2)