Cash, Cash Equivalents, and Marketable Securities - Fair Values on Recurring Basis (Details) - Recurring - USD ($) $ in Millions	9 Months Ended
Sep. 30, 2018	Dec. 31, 2017
Schedule of Investments [Line Items]
Cash	$ 8,053	$ 9,982
Total Estimated Fair Value
Cash, cash equivalents and short-term investments	30,374	32,321
Less: Restricted cash, cash equivalents, and marketable securities	(609)	(1,335)
Total cash, cash equivalents, and marketable securities	29,765	30,986
Cost or Amortized Cost
Cash, cash equivalents and short-term investments	30,283
Gross Unrealized Gains
Equity and Available For Sale Debt Securities Unrealized Gains	142
Gross Unrealized Losses
Debt Securities, Available for Sale, Unrealized Loss	(51)
Level 1 securities
Total Estimated Fair Value
Equity Securities, FV-NI	165	53
Cost or Amortized Cost
Equity Securities, FV-NI, Cost	28
Gross Unrealized Gains
Equity Securities, FV-NI, Unrealized Gain	137
Gross Unrealized Losses
Equity Securities, FV-NI, Unrealized Loss	0
Level 1 securities \| Money market funds
Schedule of Investments [Line Items]
Money market funds	11,128	11,343
Level 2 securities
Total Estimated Fair Value
Equity Securities, FV-NI	31	0
Cost or Amortized Cost
Equity Securities, FV-NI, Cost	28
Gross Unrealized Gains
Equity Securities, FV-NI, Unrealized Gain	3
Gross Unrealized Losses
Equity Securities, FV-NI, Unrealized Loss	0
Level 2 securities \| Foreign government and agency securities
Total Estimated Fair Value
Debt Securities, Available for Sale, Fair Value	481	620
Cost or Amortized Cost
Debt Securities, Available for Sale, Cost	481
Gross Unrealized Gains
Debt Securities, Available for Sale, Unrealized Gain	0
Gross Unrealized Losses
Debt Securities, Available for Sale, Unrealized Loss	0
Level 2 securities \| U.S. government and agency securities
Total Estimated Fair Value
Debt Securities, Available for Sale, Fair Value	5,519	4,823
Cost or Amortized Cost
Debt Securities, Available for Sale, Cost	5,544
Gross Unrealized Gains
Debt Securities, Available for Sale, Unrealized Gain	1
Gross Unrealized Losses
Debt Securities, Available for Sale, Unrealized Loss	(26)
Level 2 securities \| Corporate debt securities
Total Estimated Fair Value
Debt Securities, Available for Sale, Fair Value	3,990	4,257
Cost or Amortized Cost
Debt Securities, Available for Sale, Cost	4,006
Gross Unrealized Gains
Debt Securities, Available for Sale, Unrealized Gain	1
Gross Unrealized Losses
Debt Securities, Available for Sale, Unrealized Loss	(17)
Level 2 securities \| Asset-backed securities
Total Estimated Fair Value
Debt Securities, Available for Sale, Fair Value	821	905
Cost or Amortized Cost
Debt Securities, Available for Sale, Cost	826
Gross Unrealized Gains
Debt Securities, Available for Sale, Unrealized Gain	0
Gross Unrealized Losses
Debt Securities, Available for Sale, Unrealized Loss	(5)
Level 2 securities \| Other fixed income securities
Total Estimated Fair Value
Debt Securities, Available for Sale, Fair Value	186	$ 338
Cost or Amortized Cost
Debt Securities, Available for Sale, Cost	189
Gross Unrealized Gains
Debt Securities, Available for Sale, Unrealized Gain	0
Gross Unrealized Losses
Debt Securities, Available for Sale, Unrealized Loss	$ (3)

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, 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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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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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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