Marketable Securities - Reported on AOCI (Details) - USD ($) $ in Millions	Sep. 30, 2018	Dec. 31, 2017
Marketable Securities [Line Items]
Amortized Cost	$ 99	$ 99
Fair Value	99	99
Corporate debt securities
Marketable Securities [Line Items]
Amortized Cost	74	68
Fair Value	74	68
U.S. government and agency debt securities
Marketable Securities [Line Items]
Amortized Cost	3	6
Fair Value	3	6
Asset-backed debt securities
Marketable Securities [Line Items]
Amortized Cost	14	11
Fair Value	14	11
Certificates of deposit
Marketable Securities [Line Items]
Amortized Cost	7	13
Fair Value	7	13
Municipal debt securities
Marketable Securities [Line Items]
Amortized Cost	1	1
Fair Value	$ 1	$ 1

Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale).

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 45
-Paragraph 1
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=109979856&loc=d3e26610-111562

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-Publisher FASB
-Name Accounting Standards Codification
-Topic 320
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (aa)
-URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563

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- Definition

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+ References

No definition available.

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