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Investments (Tables) | 12 Months Ended
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Dec. 31, 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 December 31, 2018 and December 31, 2017 are presented below:
| December 31, 2018
(in millions) | Amortized Cost | Gross Unrealized Gains | Gross Unrealized Losses | Fair Value
Asset-backed securities | $ | 33 |  | $ | — |  | $ | — |  | $ | 33 |
Corporate bonds | 99 |  | — |  | — |  | 99 |
U.S. government agencies and government-sponsored agencies | 7 |  | — |  | — |  | 7 |
U.S. treasuries | 46 |  | — |  | — |  | 46 |
Exchange traded fund | 1 |  | — |  | — |  | 1 |
Other debt securities | 9 |  | — |  | — |  | 9 |
Total | $ | 195 |  | $ | — |  | $ | — |  | $ | 195 |
| 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 as of December 31, 2018 and December 31, 2017 are presented below.
| December 31, 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 | $ | 25 |  | $ | — |  | $ | — |  | $ | — |  | $ | 25 |  | $ | — |
Corporate bonds | 84 |  | — |  | — |  | — |  | 84 |  | — |
U.S. government agencies and government-sponsored agencies | 4 |  | — |  | — |  | — |  | 4 |  | — |
U.S. treasuries | 21 |  | — |  | — |  | — |  | 21 |  | — |
Other debt securities | 7 |  | — |  | — |  | — |  | 7 |  | — |
Total | $ | 141 |  | $ | — |  | $ | — |  | $ | — |  | $ | 141 |  | $ | — |
| 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 | Actual maturities may differ from contractual maturities because borrowers may have the right to prepay obligations with or without prepayment penalties. The fair value of debt investments by contractual maturity are shown below:
| December 31, 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 | $ | 4 |  | $ | 26 |  | $ | 3 |  | $ | — |  | $ | 33 |
Corporate bonds | 42 |  | 57 |  | — |  | — |  | 99 |
U.S. government agencies and government-sponsored agencies | 1 |  | 2 |  | — |  | 4 |  | 7 |
U.S. treasuries | 12 |  | 34 |  | — |  | — |  | 46 |
Other debt securities | — |  | 1 |  | — |  | 8 |  | 9 |
Total | $ | 59 |  | $ | 120 |  | $ | 3 |  | $ | 12 |  | $ | 194 |
| 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 |
Proceeds from Available-for-sale Securities | The gross proceeds from sales and maturities of AFS securities for the years ended December 31, 2018, 2017 and 2016 are shown below. We had immaterial gross realized gains and losses from sales of investments for the years ended December 31, 2018, 2017 and 2016.
| Year Ended December 31,
(in millions) | 2018 | 2017 | 2016
Gross proceeds from sales | $ | 54 |  | $ | — |  | $ | — |
Gross proceeds from maturities | 47 |  | 14 |  | 28 |
Total | $ | 101 |  | $ | 14 |  | $ | 28 |

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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 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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