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Investments (Tables) | 6 Months Ended
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Jun. 30, 2018
Investments, Debt and Equity Securities [Abstract] |
Amortized Cost, Gross Unrealized Gains and Losses, Fair Values of Investments | The amortized cost, gross unrealized gains, gross unrealized losses, and fair values of our investments as of June 30, 2018 and December 31, 2017 are presented below:
| June 30, 2018
(in millions) | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Fair Value
Asset-backed securities | $ | 38 |  | $ | — |  | $ | — |  | $ | 38 |
Corporate bonds | 87 |  | — |  | — |  | 87 |
U.S. government agencies and government- sponsored agencies | 6 |  | — |  | — |  | 6 |
U.S. treasuries | 34 |  | — |  | — |  | 34 |
Exchange traded fund | 1 |  | — |  | — |  | 1 |
Other debt securities | 11 |  | — |  | — |  | 11 |
Total | $ | 177 |  | $ | — |  | $ | — |  | $ | 177 |
| December 31, 2017
(in millions) | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Fair Value
U.S. treasuries | $ | 37 |  | $ | — |  | $ | — |  | $ | 37 |
Exchange traded fund | 1 |  | — |  | — |  | 1 |
Total | $ | 38 |  | $ | — |  | $ | — |  | $ | 38 |
Investments with a Continuous Unrealized Loss Position | Investments in a continuous unrealized loss position for less than 12 months and 12 months or more as of June 30, 2018 and December 31, 2017 are presented below.
| June 30, 2018
| Less than 12 months | 12 months or more | Total
(in millions) | Fair Value | Unrealized Losses | Fair Value | Unrealized Losses | Fair Value | Unrealized Losses
Asset-backed securities | $ | 26 |  | $ | — |  | $ | — |  | $ | — |  | $ | 26 |  | $ | — |
Corporate bonds | 82 |  | — |  | — |  | — |  | 82 |  | — |
U.S. government agencies and government-sponsored agencies | 5 |  | — |  | — |  | — |  | 5 |  | — |
U.S. treasuries | 20 |  | — |  | 10 |  | — |  | 30 |  | — |
Other debt securities | 2 |  | — |  | — |  | — |  | 2 |  | — |
Total | $ | 135 |  | $ | — |  | $ | 10 |  | $ | — |  | $ | 145 |  | $ | — |
| December 31, 2017
| Less than 12 months | 12 months or more | Total
(in millions) | Fair Value | Unrealized Losses | Fair Value | Unrealized Losses | Fair Value | Unrealized Losses
U.S. treasuries | $ | 5 |  | $ | — |  | $ | 24 |  | $ | — |  | $ | 29 |  | $ | — |
Total | $ | 5 |  | $ | — |  | $ | 24 |  | $ | — |  | $ | 29 |  | $ | — |
Fair Value of Debt Investments by Contractual Maturity | The fair value of debt investments by contractual maturity (actual maturities may differ from contractual maturities because borrowers may have the right to prepay obligations with or without prepayment penalties) are shown below.
| June 30, 2018
(in millions) | One year or less | Over One Year Through Five Years | Over Five Years Through Ten Years | Over Ten Years | Fair Value
Asset-backed securities | $ | 2 |  | $ | 32 |  | $ | 3 |  | $ | 1 |  | $ | 38 |
Corporate bonds | 24 |  | 63 |  | — |  | — |  | 87 |
U.S. government agencies and government-sponsored agencies | 2 |  | — |  | — |  | 4 |  | 6 |
U.S. treasuries | 2 |  | 32 |  | — |  | — |  | 34 |
Other debt securities | — |  | — |  | — |  | 11 |  | 11 |
Total | $ | 30 |  | $ | 127 |  | $ | 3 |  | $ | 16 |  | $ | 176 |
| December 31, 2017
(in millions) | One year or less | Over One Year Through Five Years | Over Five Years Through Ten Years | Over Ten Years | Fair Value
U.S. treasuries | $ | — |  | $ | 37 |  | $ | — |  | $ | — |  | $ | 37 |
Total | $ | — |  | $ | 37 |  | $ | — |  | $ | — |  | $ | 37 |
Gross Realized Gains and Losses from Sales of Investments | There were immaterial gross realized gains and losses from sales of investments for the three and six months periods ended June 30, 2018 and 2017.

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- Definition Tabular disclosure of fair value amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in unrealized loss position. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition Tabular disclosure of maturities of an entity's investments as well as any other information pertinent to the investments. + References No definition available. + Details
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- Definition Tabular disclosure of the proceeds from sales of available-for-sale securities and the gross realized gains and gross realized losses that have been included in earnings as a result of those sales. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 9 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27357-111563 + Details
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