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Cash, Cash Equivalents and Investments - Amortized Cost, Gross Unrealized Gains and Losses, Fair Values and Related Maturities of Securities Available for Sale (Details) - USD ($) $ in Millions | 3 Months Ended | 12 Months Ended
--- | --- | ---
Mar. 31, 2018 | Dec. 31, 2017
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized Cost | $ 24 | $ 38
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | 24 | 38
U.S. treasuries |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized Cost | 23 | 37
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | $ 23 | $ 37
U.S. treasuries | Minimum |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Maturity | 1 year | 1 year
U.S. treasuries | Maximum |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Maturity | 5 years | 5 years
Exchange traded fund |  |
Schedule Of Available For Sale Securities [Line Items] |  |
Amortized Cost | $ 1 | $ 1
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | 0 | 0
Fair Value | $ 1 | $ 1

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- 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) -URI http://asc.fasb.org/extlink&oid=75030427&loc=d3e22054-111558 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 5 -Subparagraph (aa) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27232-111563 Reference 3: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 45 -Paragraph 1 -URI http://asc.fasb.org/extlink&oid=82896770&loc=d3e26610-111562 + Details
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