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Investments - (Tables) | 6 Months Ended
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Jun. 30, 2019
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, 2019 and December 31, 2018 are presented below.
| June 30, 2019 | December 31, 2018
(in millions) | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Fair Value | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Fair Value
Asset-backed securities | $ | 35 |  | $ | — |  | $ | — |  | $ | 35 |  | $ | 33 |  | $ | — |  | $ | — |  | $ | 33 |
Corporate bonds | 94 |  | 1 |  | — |  | 95 |  | 99 |  | — |  | — |  | 99 |
U.S. government agencies and government- sponsored agencies | 5 |  | — |  | — |  | 5 |  | 7 |  | — |  | — |  | 7 |
U.S. treasuries | 58 |  | 1 |  | — |  | 59 |  | 46 |  | — |  | — |  | 46 |
Exchange traded fund | 1 |  | — |  | — |  | 1 |  | 1 |  | — |  | — |  | 1 |
Other debt securities | 4 |  | — |  | — |  | 4 |  | 9 |  | — |  | — |  | 9 |
Total | $ | 197 |  | $ | 2 |  | $ | — |  | $ | 199 |  | $ | 195 |  | $ | — |  | $ | — |  | $ | 195 |
Fair Value of Debt Investments by Contractual Maturity | The fair value of debt investments by contractual maturity are shown below:
(in millions) |  | June 30, 2019 |
One year or less |  | $ | 81 |  |
Over one year through five years |  | 103 |  |
Over five years through ten years |  | 7 |  |
Over ten years |  | 7 |  |
Total fair value |  | $ | 198 |  |
Proceeds from available-for-sale securities | The gross proceeds from sales and maturities of AFS securities for the three and six months ended June 30, 2019 and June 30, 2018 are presented below. We had immaterial realized gains and losses from sales of investments for the three and six months ended June 30, 2019 and June 30, 2018.
| Three Months Ended June 30, | Six Months Ended June 30,
(in millions) | 2019 | 2018 | 2019 | 2018
Gross proceeds from sales | $ | 14 |  | $ | — |  | $ | 28 |  | $ | 39 |
Gross proceeds from maturities | 19 |  | 10 |  | 36 |  | 24 |

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- Definition Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 + 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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