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Investments | 12 Months Ended
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Dec. 31, 2017
Investments, Debt and Equity Securities [Abstract] |
Investments | Investments We consider all of our investments, and our restricted investments that are classified as current assets, to be available-for-sale. As described further in Note 11, “Debt,” we maintain certain funds from the issuance of our 4.875% Notes in a segregated deposit account, a current asset reported as “Restricted investments” in the accompanying consolidated balance sheets. Such investments, while restricted as to their use and held in a segregated deposit account, are classified as available-for-sale based upon our contractual liquidity requirements. 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 |
There were no current restricted investments as of December 31, 2016. 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 |
Gross realized gains and losses from sales of available-for-sale securities are calculated under the specific identification method and are included in investment income. Gross realized investment gains and losses for the years ended December 31, 2017, 2016 and 2015 were insignificant. We have determined that unrealized losses at December 31, 2017 and 2016 are temporary in nature, because the change in market value for these securities resulted from fluctuating interest rates, rather than a deterioration of the creditworthiness of the issuers. So long as we maintain the intent and ability to hold these securities to maturity, we are unlikely to experience losses. In the event that we dispose of these securities before maturity, we expect that realized losses, if any, will be insignificant. 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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