Fair Value Measurements - Fair Value of Investment Assets (Details) - USD ($) $ in Millions	Jun. 30, 2019	Dec. 31, 2018
Carrying Value
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Amortized cost basis of investments	$ 957	$ 340
Fair Value \| Level 2
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	964	341
Fair Value \| Level 2 \| Certificates of Deposit and Commercial Paper
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	869	250
Fair Value \| Level 2 \| Corporate Bonds
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	61	56
Fair Value \| Level 2 \| Government Securities
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items]
Investments at fair value	$ 34	$ 35

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=118939692&loc=d3e27161-111563

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 326
-SubTopic 30
-Section 45
-Paragraph 1
-URI http://asc.fasb.org/extlink&oid=118942338&loc=SL82898722-210454

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+ References

No definition available.

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- Definition

Fair value portion of investment securities, including, but not limited to, marketable securities, derivative financial instruments, and investments accounted for under the equity method.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 820
-SubTopic 10
-Section 50
-Paragraph 2
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=117815213&loc=d3e19207-110258

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