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Investments (Tables) | 12 Months Ended
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Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract] |
Investments | The following tables summarize our investments as of the dates indicated:
| December 31, 2016
| Amortized |  | Gross Unrealized |  | Estimated
| Cost |  | Gains |  | Losses |  | Fair Value
| (In millions)
Corporate debt securities | $ | 1,180 |  |  | $ | 1 |  |  | $ | 2 |  |  | $ | 1,179 |
GSEs | 232 |  |  | — |  |  | 1 |  |  | 231 |
Municipal securities | 143 |  |  | — |  |  | 1 |  |  | 142 |
U.S. treasury notes | 84 |  |  | — |  |  | — |  |  | 84 |
Asset-backed securities | 69 |  |  | — |  |  | — |  |  | 69 |
Certificates of deposit | 53 |  |  | — |  |  | — |  |  | 53 |
| $ | 1,761 |  |  | $ | 1 |  |  | $ | 4 |  |  | $ | 1,758 |
| December 31, 2015
| Amortized Cost |  | Gross Unrealized |  | Estimated Fair Value
|  | Gains |  | Losses |
| (In millions)
Corporate debt securities | $ | 1,189 |  |  | $ | — |  |  | $ | 5 |  |  | $ | 1,184 |
GSEs | 212 |  |  | — |  |  | 1 |  |  | 211 |
Municipal securities | 186 |  |  | — |  |  | 1 |  |  | 185 |
U.S. treasury notes | 78 |  |  | — |  |  | — |  |  | 78 |
Asset-backed securities | 63 |  |  | — |  |  | — |  |  | 63 |
Certificates of deposit | 80 |  |  | — |  |  | — |  |  | 80 |
| $ | 1,808 |  |  | $ | — |  |  | $ | 7 |  |  | $ | 1,801 |
Contractual maturities of investments | The contractual maturities of our investments as of December 31, 2016 are summarized below:
| Amortized Cost |  | Estimated Fair Value
| (In millions)
Due in one year or less | $ | 930 |  |  | $ | 930 |
Due after one year through five years | 806 |  |  | 803 |
Due after five years through ten years | 25 |  |  | 25 |
| $ | 1,761 |  |  | $ | 1,758 |
Available-for-sale investments | The following table segregates those available-for-sale investments that have been in a continuous loss position for less than 12 months, and those that have been in a loss position for 12 months or more as of December 31, 2016.
| In a Continuous Loss Position for Less than 12 Months |  | In a Continuous Loss Position for 12 Months or More
| Estimated Fair Value |  | Unrealized Losses |  | Total Number of Positions |  | Estimated Fair Value |  | Unrealized Losses |  | Total Number of Positions
| (Dollars in millions)
Corporate debt securities | $ | 542 |  |  | $ | 2 |  |  | 378 |  |  | $ | — |  |  | $ | — |  |  | — |
GSEs | 198 |  |  | 1 |  |  | 73 |  |  | — |  |  | — |  |  | — |
Municipal securities | 101 |  |  | 1 |  |  | 129 |  |  | — |  |  | — |  |  | — |
| $ | 841 |  |  | $ | 4 |  |  | 580 |  |  | $ | — |  |  | $ | — |  |  | — |
The following table segregates those available-for-sale investments that have been in a continuous loss position for less than 12 months, and those that have been in a loss position for 12 months or more as of December 31, 2015.
| In a Continuous Loss Position for Less than 12 Months |  | In a Continuous Loss Position for 12 Months or More
| Estimated Fair Value |  | Unrealized Losses |  | Total Number of Positions |  | Estimated Fair Value |  | Unrealized Losses |  | Total Number of Positions
| (Dollars in millions)
Corporate debt securities | $ | 825 |  |  | $ | 4 |  |  | 588 |  |  | $ | 119 |  |  | $ | 1 |  |  | 87 |
GSEs | 182 |  |  | 1 |  |  | 77 |  |  | — |  |  | — |  |  | — |
Municipal securities | 128 |  |  | 1 |  |  | 181 |  |  | — |  |  | — |  |  | — |
| $ | 1,135 |  |  | $ | 6 |  |  | 846 |  |  | $ | 119 |  |  | $ | 1 |  |  | 87 |

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- Definition Tabular disclosure of fair value of investments in debt and equity securities in an unrealized loss position categorized neither as held-to-maturity nor trading securities. + References No definition available. + Details
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