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Investments - Fair Value of Debt Investments by Contractual Maturity (Details) - USD ($) $ in Millions | Sep. 30, 2018 | Dec. 31, 2017
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Debt Securities, Available-for-sale [Line Items] |  |
One year or less | $ 43 | $ 0
Over One Year Through Five Years | 113 | 37
Over Five Years Through Ten Years | 3 | 0
Over Ten Years | 14 | 0
Fair Value | 173 | 37
Asset-backed securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
One year or less | 3 |
Over One Year Through Five Years | 29 |
Over Five Years Through Ten Years | 3 |
Over Ten Years | 0 |
Fair Value | 35 |
Corporate bonds |  |
Debt Securities, Available-for-sale [Line Items] |  |
One year or less | 32 |
Over One Year Through Five Years | 54 |
Over Five Years Through Ten Years | 0 |
Over Ten Years | 0 |
Fair Value | 86 |
U.S. government agencies and government- sponsored agencies |  |
Debt Securities, Available-for-sale [Line Items] |  |
One year or less | 1 |
Over One Year Through Five Years | 2 |
Over Five Years Through Ten Years | 0 |
Over Ten Years | 5 |
Fair Value | 8 |
U.S. treasuries |  |
Debt Securities, Available-for-sale [Line Items] |  |
One year or less | 7 | 0
Over One Year Through Five Years | 27 | 37
Over Five Years Through Ten Years | 0 | 0
Over Ten Years | 0 | 0
Fair Value | 34 | $ 37
Other debt securities |  |
Debt Securities, Available-for-sale [Line Items] |  |
One year or less | 0 |
Over One Year Through Five Years | 1 |
Over Five Years Through Ten Years | 0 |
Over Ten Years | 9 |
Fair Value | $ 10 |

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- Definition Fair value of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), with single maturity date and allocated without single maturity date, maturing in second through fifth fiscal year following latest fiscal year. Excludes interim and annual periods when interim period is reported on rolling approach, from latest statement of financial position date. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27198-111563 Reference 2: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 942 -SubTopic 320 -Section 50 -Paragraph 3 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=75019621&loc=d3e62586-112803 Reference 3: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (d) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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