Financial Instruments - Fair Values on Recurring Basis (Details) - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2019	Dec. 31, 2018
Schedule of Investments [Line Items]
Equity Securities, FV-NI, Unrealized Gain (Loss)	$ 68
Recurring
Schedule of Investments [Line Items]
Cash	8,635	$ 10,406
Total Estimated Fair Value
Cash, cash equivalents and marketable securities	37,414	41,676
Less: Restricted cash, cash equivalents, and marketable securities	(394)	(426)
Total cash, cash equivalents, and marketable securities	37,020	41,250
Cost or Amortized Cost
Cash, cash equivalents and marketable securities	37,159
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain (Loss), before Tax
Debt Securities, Available for Sale, Unrealized Gain	14
Debt Securities, Available for Sale, Unrealized Loss	(22)
Recurring \| Level 1 securities
Total Estimated Fair Value
Equity Securities, FV-NI	263	170
Recurring \| Level 1 securities \| Money market funds
Schedule of Investments [Line Items]
Money market funds	9,515	12,515
Recurring \| Level 2 securities
Total Estimated Fair Value
Equity Securities, FV-NI	0	33
Recurring \| Level 2 securities \| Foreign government and agency securities
Total Estimated Fair Value
Debt Securities, Available for Sale, Fair Value	1,382	815
Cost or Amortized Cost
Debt Securities, Available for Sale, Cost	1,382
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain (Loss), before Tax
Debt Securities, Available for Sale, Unrealized Gain	0
Debt Securities, Available for Sale, Unrealized Loss	0
Recurring \| Level 2 securities \| U.S. government and agency securities
Total Estimated Fair Value
Debt Securities, Available for Sale, Fair Value	8,854	11,667
Cost or Amortized Cost
Debt Securities, Available for Sale, Cost	8,864
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain (Loss), before Tax
Debt Securities, Available for Sale, Unrealized Gain	3
Debt Securities, Available for Sale, Unrealized Loss	(13)
Recurring \| Level 2 securities \| Corporate debt securities
Total Estimated Fair Value
Debt Securities, Available for Sale, Fair Value	7,398	4,990
Cost or Amortized Cost
Debt Securities, Available for Sale, Cost	7,396
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain (Loss), before Tax
Debt Securities, Available for Sale, Unrealized Gain	8
Debt Securities, Available for Sale, Unrealized Loss	(6)
Recurring \| Level 2 securities \| Asset-backed securities
Total Estimated Fair Value
Debt Securities, Available for Sale, Fair Value	1,167	892
Cost or Amortized Cost
Debt Securities, Available for Sale, Cost	1,166
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain (Loss), before Tax
Debt Securities, Available for Sale, Unrealized Gain	3
Debt Securities, Available for Sale, Unrealized Loss	(2)
Recurring \| Level 2 securities \| Other fixed income securities
Total Estimated Fair Value
Debt Securities, Available for Sale, Fair Value	200	$ 188
Cost or Amortized Cost
Debt Securities, Available for Sale, Cost	201
Debt Securities, Available-for-sale, Accumulated Gross Unrealized Gain (Loss), before Tax
Debt Securities, Available for Sale, Unrealized Gain	0
Debt Securities, Available for Sale, Unrealized Loss	$ (1)

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

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Cash, Cash Equivalents, And Marketable Securities

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Cash, Cash Equivalents, And Marketable Securities, Excluding Restricted Cash And Investments

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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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