Investments - Estimated Fair Values of Investments Classified as Available for Sale (Details) - USD ($) $ in Millions	Mar. 31, 2018	Dec. 31, 2017
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	$ 4,247	$ 4,169
Gross Unrealized Gains	1	3
Gross Unrealized Losses	(21)	(8)
Estimated Fair Value	4,227	4,164
Short-term restricted cash	81
Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Gross Unrealized Losses	(3,400)	(2,800)
Short-term investments
Schedule of Available-for-sale Securities [Line Items]
Short-term restricted cash		79
Short-term investments \| Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	2,340	2,092
Gross Unrealized Gains	0	1
Gross Unrealized Losses	(4)	(1)
Estimated Fair Value	2,336	2,092
Short-term investments \| Government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	495	210
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(1)	0
Estimated Fair Value	494	210
Short-term investments \| Time deposits
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	199	163
Long-term investments
Schedule of Available-for-sale Securities [Line Items]
Long-term restricted cash	2	2
Long-term investments \| Corporate debt securities
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	1,324	1,769
Gross Unrealized Gains	1	2
Gross Unrealized Losses	(16)	(7)
Estimated Fair Value	1,309	1,764
Long-term investments \| Government and agency securities
Schedule of Available-for-sale Securities [Line Items]
Gross Amortized Cost	88	98
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Estimated Fair Value	$ 88	$ 98

Amount of investment in debt and equity securities categorized neither as held-to-maturity nor trading.

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