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Fair Value Measurements - Fair Value of Investment Assets (Details) - USD ($) $ in Millions | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Carrying Value |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Amortized cost basis of investments | $ 903 | $ 340
Fair Value | Level 2 |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Investments at fair value | 906 | 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 | 813 | 250
Fair Value | Level 2 | Corporate Bonds |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Investments at fair value | 59 | 56
Fair Value | Level 2 | Government Securities |  |
Fair Value, Balance Sheet Grouping, Financial Statement Captions [Line Items] |  |
Investments at fair value | $ 34 | $ 35

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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=99377789&loc=d3e19207-110258 + Details
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