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Financial Instruments - Information on Available-for-sale Investments (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
	Sep. 30, 2018	Sep. 30, 2018	Dec. 31, 2017
Debt Securities, Available-for-sale [Line Items]
Debt securities, fair value	$ 10,176	$ 10,176	$ 14,908
Publicly traded equity securities, fair value	378	378
Publicly traded equity securities, fair value			275
Total debt and publicly traded equity securities, fair value	10,554	10,554
Total debt and publicly traded equity securities, fair value			15,183
Debt securities, amortized cost	10,300	10,300	14,976
Publicly traded equity securities, amortized cost			265
Total debt and publicly traded equity securities, amortized cost			15,241
Debt securities, unrealized gains	3	3	11
Equity securities, unrealized gains			16
Total debt and publicly traded equity securities, unrealized gains			27
Debt securities, unrealized losses	(127)	(127)	(79)
Equity securities, unrealized losses			(6)
Total debt and publicly traded equity securities, unrealized losses			(85)
Unrealized net gain	10	60
Commercial paper
Debt Securities, Available-for-sale [Line Items]
Debt securities, fair value	30	30	159
Debt securities, amortized cost	30	30	159
Debt securities, unrealized gains	0	0	0
Debt securities, unrealized losses	0	0	0
Corporate notes and bonds
Debt Securities, Available-for-sale [Line Items]
Debt securities, fair value	6,866	6,866	9,806
Debt securities, amortized cost	6,952	6,952	9,837
Debt securities, unrealized gains	2	2	9
Debt securities, unrealized losses	(88)	(88)	(40)
U.S. government and agency securities
Debt Securities, Available-for-sale [Line Items]
Debt securities, fair value	1,431	1,431	2,042
Debt securities, amortized cost	1,448	1,448	2,059
Debt securities, unrealized gains	0	0	0
Debt securities, unrealized losses	(17)	(17)	(17)
Asset-backed securities
Debt Securities, Available-for-sale [Line Items]
Debt securities, fair value	1,288	1,288	1,542
Debt securities, amortized cost	1,300	1,300	1,548
Debt securities, unrealized gains	1	1	1
Debt securities, unrealized losses	(13)	(13)	(7)
Foreign government bonds
Debt Securities, Available-for-sale [Line Items]
Debt securities, fair value	538	538	733
Debt securities, amortized cost	546	546	739
Debt securities, unrealized gains	0	0	0
Debt securities, unrealized losses	(8)	(8)	(6)
Mortgage-backed securities
Debt Securities, Available-for-sale [Line Items]
Debt securities, fair value	23	23	626
Debt securities, amortized cost	24	24	634
Debt securities, unrealized gains	0	0	1
Debt securities, unrealized losses	$ (1)	$ (1)	$ (9)

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Debt Securities and Equity Securities, FV-NI

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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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