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Investments (Tables) | 12 Months Ended
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Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |
Investments | The following tables summarize our investments as of the dates indicated:
| December 31, 2017
| Amortized |  | Gross Unrealized |  | Estimated
| Cost |  | Gains |  | Losses |  | Fair Value
| (In millions)
Corporate debt securities | $ | 1,591 |  |  | $ | 1 |  |  | $ | 4 |  |  | $ | 1,588 |
U.S. Treasury notes | 389 |  |  | — |  |  | 1 |  |  | 388 |
GSEs | 255 |  |  | — |  |  | 2 |  |  | 253 |
Municipal securities | 142 |  |  | — |  |  | 1 |  |  | 141 |
Asset-backed securities | 117 |  |  | — |  |  | — |  |  | 117 |
Certificates of deposit | 37 |  |  | — |  |  | — |  |  | 37 |
Subtotal - current investments | 2,531 |  |  | 1 |  |  | 8 |  |  | 2,524 |
Corporate debt securities | 101 |  |  | — |  |  | — |  |  | 101 |
U.S. treasury notes | 68 |  |  | — |  |  | — |  |  | 68 |
Subtotal - restricted investments, current | 169 |  |  | — |  |  | — |  |  | 169 |
| $ | 2,700 |  |  | $ | 1 |  |  | $ | 8 |  |  | $ | 2,693 |
| December 31, 2016
| Amortized Cost |  | Gross Unrealized |  | Estimated Fair Value
|  | Gains |  | Losses |
| (In millions)
Corporate debt securities | $ | 1,180 |  |  | $ | 1 |  |  | $ | 2 |  |  | $ | 1,179 |
U.S. treasury notes | 84 |  |  | — |  |  | — |  |  | 84 |
GSEs | 232 |  |  | — |  |  | 1 |  |  | 231 |
Municipal securities | 143 |  |  | — |  |  | 1 |  |  | 142 |
Asset-backed securities | 69 |  |  | — |  |  | — |  |  | 69 |
Certificates of deposit | 53 |  |  | — |  |  | — |  |  | 53 |
| $ | 1,761 |  |  | $ | 1 |  |  | $ | 4 |  |  | $ | 1,758 |
Contractual maturities of investments | The contractual maturities of our available-for-sale investments as of December 31, 2017 are summarized below:
| Amortized Cost |  | Estimated Fair Value
| (In millions)
Due in one year or less | $ | 1,722 |  |  | $ | 1,721 |
Due after one year through five years | 972 |  |  | 966 |
Due after five years through ten years | 6 |  |  | 6 |
| $ | 2,700 |  |  | $ | 2,693 |
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 continuous loss position for 12 months or more as of December 31, 2017.
| 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 | $ | 1,297 |  |  | $ | 3 |  |  | 561 |  |  | $ | 94 |  |  | $ | 1 |  |  | 69 |
U.S. Treasury notes | 470 |  |  | 1 |  |  | 89 |  |  | — |  |  | — |  |  | — |
GSEs | 173 |  |  | 1 |  |  | 69 |  |  | 95 |  |  | 1 |  |  | 47 |
Municipal securities | — |  |  | — |  |  | — |  |  | 38 |  |  | 1 |  |  | 48 |
| $ | 1,940 |  |  | $ | 5 |  |  | 719 |  |  | $ | 227 |  |  | $ | 3 |  |  | 164 |
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 continuous 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 |  |  | $ | — |  |  | $ | — |  |  | — |

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