v3.8.0.1

Short-term and Long-term Investments, Restricted Deposits - Schedule by investment Type (Details) - USD ($) $ in Millions	Dec. 31, 2017	Dec. 31, 2016
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	$ 5,982	$ 5,233
Gross Unrealized Gains	27	15
Gross Unrealized Losses	(31)	(60)
Fair Value	5,978	5,188
U.S. Treasury securities and obligations of U.S. government corporations and agencies
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	311	364
Gross Unrealized Gains	0	0
Gross Unrealized Losses	(2)	(1)
Fair Value	309	363
Corporate securities
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	2,208	1,933
Gross Unrealized Gains	12	12
Gross Unrealized Losses	(10)	(13)
Fair Value	2,210	1,932
Restricted certificates of deposit
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	4	5
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	4	5
Restricted cash equivalents
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	17	6
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	17	6
Municipal securities
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	2,085	1,767
Gross Unrealized Gains	12	1
Gross Unrealized Losses	(10)	(35)
Fair Value	2,087	1,733
Asset-backed securities
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	437	317
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(1)	(1)
Fair Value	437	317
Residential mortgage-backed securities
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	337	219
Gross Unrealized Gains	1	1
Gross Unrealized Losses	(6)	(5)
Fair Value	332	215
Commercial mortgage- backed securities
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	272	343
Gross Unrealized Gains	1	0
Gross Unrealized Losses	(2)	(5)
Fair Value	271	338
Equity method investments
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	176	163
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	176	163
Life insurance contracts
Schedule Of Investments And Restricted Deposits By Type [Line Items]
Amortized Cost	135	116
Gross Unrealized Gains	0	0
Gross Unrealized Losses	0	0
Fair Value	$ 135	$ 116

X

- Definition

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

+ References

Reference 1: http://www.xbrl.org/2003/role/presentationRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 25
-Paragraph 1
-Subparagraph (b)
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+ References

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
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-Section 50
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+ References

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+ References

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-Name Accounting Standards Codification
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