Investments (Schedule of Held-to-Maturity Securities) (Details) - USD ($) $ in Millions	Mar. 31, 2019	Dec. 31, 2018
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost, Current	$ 826	$ 1,065
Amortized Cost, Noncurrent	6	6
Unrealized Gains	2	0
Unrealized Losses	0	(1)
Fair Value	834	1,070
Amortized Cost, held-to-maturity	832	1,071
Less than 12 Months, Fair Value	121	531
Less than 12 Months, Unrealized Losses	0	(1)
12 Months or More, Fair Value	6	57
12 Months or More, Unrealized Losses	0	0
Fair Value	127	588
Unrealized Losses	0	(1)
Corporate And Other Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than 12 Months, Fair Value	29	263
Less than 12 Months, Unrealized Losses	0	0
12 Months or More, Fair Value	6	57
12 Months or More, Unrealized Losses	0	0
Fair Value	35	320
Unrealized Losses	0	0
Commercial Paper [Member]
Schedule of Held-to-maturity Securities [Line Items]
Less than 12 Months, Fair Value	92	268
Less than 12 Months, Unrealized Losses	0	(1)
12 Months or More, Fair Value	0	0
12 Months or More, Unrealized Losses	0	0
Fair Value	92	268
Unrealized Losses	0	(1)
Short-term Investments [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost, Current	826	1,065
Unrealized Gains	2	0
Unrealized Losses	0	(1)
Fair Value	828	1,064
Short-term Investments [Member] \| Corporate And Other Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost, Current	417	546
Unrealized Gains	1	0
Unrealized Losses	0	0
Fair Value	418	546
Short-term Investments [Member] \| Commercial Paper [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost, Current	409	519
Unrealized Gains	1	0
Unrealized Losses	0	(1)
Fair Value	410	518
Long-term investments [Member] \| Corporate And Other Debt Securities [Member]
Schedule of Held-to-maturity Securities [Line Items]
Amortized Cost, Noncurrent	6	6
Unrealized Gains	0	0
Unrealized Losses	0	0
Fair Value	$ 6	$ 6

Amount of investment in debt security measured at amortized cost (held-to-maturity).

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