Cash, Cash Equivalents, and Marketable Securities - Fair Values on Recurring Basis (Details) - Recurring - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Cash	$ 10,406	$ 9,982
Cost or Amortized Cost
Cash, cash equivalents and short-term investments	41,573	32,324
Gross Unrealized Gains
Short-term investments	150	32
Gross Unrealized Losses
Cash equivalents and marketable securities	(47)	(35)
Total Estimated Fair Value
Cash, cash equivalents and short-term investments	41,676	32,321
Less: Restricted cash, cash equivalents, and marketable securities	(426)	(1,335)
Total cash, cash equivalents, and marketable securities	41,250	30,986
Level 1 securities
Cost or Amortized Cost
Equity securities	29	23
Gross Unrealized Gains
Equity securities	143	30
Gross Unrealized Losses
Equity securities	(2)	0
Total Estimated Fair Value
Equity securities	170	53
Level 1 securities \| Money market funds
Schedule of Investments [Line Items]
Money market funds	12,515	11,343
Level 2 securities
Cost or Amortized Cost
Equity securities	28
Gross Unrealized Gains
Equity securities	5
Gross Unrealized Losses
Equity securities	0
Total Estimated Fair Value
Equity securities	33
Level 2 securities \| Foreign government and agency securities
Cost or Amortized Cost
Cash equivalents and marketable securities	815	620
Gross Unrealized Gains
Cash equivalents and marketable securities	0	0
Gross Unrealized Losses
Cash equivalents and marketable securities	0	0
Total Estimated Fair Value
Cash equivalents and marketable securities	815	620
Level 2 securities \| U.S. government and agency securities
Cost or Amortized Cost
Cash equivalents and marketable securities	11,686	4,841
Gross Unrealized Gains
Cash equivalents and marketable securities	1	1
Gross Unrealized Losses
Cash equivalents and marketable securities	(20)	(19)
Total Estimated Fair Value
Cash equivalents and marketable securities	11,667	4,823
Level 2 securities \| Corporate debt securities
Cost or Amortized Cost
Cash equivalents and marketable securities	5,008	4,265
Gross Unrealized Gains
Cash equivalents and marketable securities	1	1
Gross Unrealized Losses
Cash equivalents and marketable securities	(19)	(9)
Total Estimated Fair Value
Cash equivalents and marketable securities	4,990	4,257
Level 2 securities \| Asset-backed securities
Cost or Amortized Cost
Cash equivalents and marketable securities	896	910
Gross Unrealized Gains
Cash equivalents and marketable securities	0	0
Gross Unrealized Losses
Cash equivalents and marketable securities	(4)	(5)
Total Estimated Fair Value
Cash equivalents and marketable securities	892	905
Level 2 securities \| Other fixed income securities
Cost or Amortized Cost
Cash equivalents and marketable securities	190	340
Gross Unrealized Gains
Cash equivalents and marketable securities	0	0
Gross Unrealized Losses
Cash equivalents and marketable securities	(2)	(2)
Total Estimated Fair Value
Cash equivalents and marketable securities	$ 188	$ 338

Cash, Cash Equivalents And Debt Available-For-Sale Securities, Amortized Cost Basis [Abstract]

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Cash, Cash Equivalents, And Short-Term Investments, Amortized Cost Basis

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Cash, Cash Equivalents, And Short-Term Investments, Current And Noncurrent

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Cash, Cash Equivalents, And Short-Term Investments, Excluding Restricted Cash And Investments

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Equity Securities, FV-NI, and Debt Securities, Available-for-sale-Accumulated Gross Unrealized Gain, before tax Amount of unrealized gain on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI) and amount, before tax, of unrealized gain on debt securities in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized gain in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

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Amount of currency on hand as well as demand deposits with banks or financial institutions. Includes other kinds of accounts that have the general characteristics of demand deposits. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of short-term, highly liquid investments that are both readily convertible to known amounts of cash and so near their maturity that they present insignificant risk of changes in value because of changes in interest rates. Excludes cash and cash equivalents within disposal group and discontinued operation.

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Amount of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Cost of investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI). Excludes equity method investment and investment in equity security without readily determinable fair value.

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Amount of unrealized loss on investment in equity security measured at fair value with change in fair value recognized in net income (FV-NI).

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Cash and investments whose use in whole or in part is restricted for the long-term, generally by contractual agreements or regulatory requirements. For use in an unclassified balance sheet.

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Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

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