Cash, Cash Equivalents, and Marketable Securities - Fair Values on Recurring Basis (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Total Estimated Fair Value
Cash equivalents and marketable securities	$ 17,087
Cost or Amortized Cost
Cash equivalents and marketable securities	17,141
Recurring
Schedule of Investments [Line Items]
Cash	8,706	$ 9,982
Total Estimated Fair Value
Cash, cash equivalents and short-term investments	25,903	32,321
Less: Restricted cash, cash equivalents, and marketable securities	(940)	(1,335)
Total cash, cash equivalents, and marketable securities	24,963	30,986
Cost or Amortized Cost
Cash, cash equivalents and short-term investments	25,898
Gross Unrealized Gains
Short-term investments	60
Gross Unrealized Losses
Short-term investments	(55)
Recurring | Level 1 securities
Total Estimated Fair Value
Equity securities	77	53
Cost or Amortized Cost
Equity securities	23
Gross Unrealized Gains
Equity securities	54
Gross Unrealized Losses
Equity securities	0
Recurring | Level 1 securities | Money market funds
Schedule of Investments [Line Items]
Money market funds	8,663	11,343
Recurring | Level 2 securities
Total Estimated Fair Value
Equity securities	33	0
Cost or Amortized Cost
Equity securities	28
Gross Unrealized Gains
Equity securities	5
Gross Unrealized Losses
Equity securities	0
Recurring | Level 2 securities | Foreign government and agency securities
Total Estimated Fair Value
Cash equivalents and marketable securities	420	620
Cost or Amortized Cost
Cash equivalents and marketable securities	421
Gross Unrealized Gains
Cash equivalents and marketable securities	0
Gross Unrealized Losses
Cash equivalents and marketable securities	(1)
Recurring | Level 2 securities | U.S. government and agency securities
Total Estimated Fair Value
Cash equivalents and marketable securities	3,533	4,823
Cost or Amortized Cost
Cash equivalents and marketable securities	3,557
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	3,393	4,257
Cost or Amortized Cost
Cash equivalents and marketable securities	3,412
Gross Unrealized Gains
Cash equivalents and marketable securities	0
Gross Unrealized Losses
Cash equivalents and marketable securities	(19)
Recurring | Level 2 securities | Asset-backed securities
Total Estimated Fair Value
Cash equivalents and marketable securities	795	905
Cost or Amortized Cost
Cash equivalents and marketable securities	803
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	283	$ 338
Cost or Amortized Cost
Cash equivalents and marketable securities	285
Gross Unrealized Gains
Cash equivalents and marketable securities	0
Gross Unrealized Losses
Cash equivalents and marketable securities	$ (2)