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Investments | 12 Months Ended
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Dec. 31, 2018
Investments, Debt and Equity Securities [Abstract] |
Investments | Investments We consider all of our investments classified as current assets to be available-for-sale. The following tables summarize our current investments as of the dates indicated:
| December 31, 2018
| Amortized |  | Gross Unrealized |  | Estimated
| Cost |  | Gains |  | Losses |  | Fair Value
| (In millions)
Corporate debt securities | $ | 1,131 |  |  | $ | — |  |  | $ | 8 |  |  | $ | 1,123 |
U.S. Treasury notes | 181 |  |  | — |  |  | — |  |  | 181 |
GSEs | 164 |  |  | — |  |  | 1 |  |  | 163 |
Municipal securities | 115 |  |  | — |  |  | 1 |  |  | 114 |
Asset-backed securities | 83 |  |  | — |  |  | 1 |  |  | 82 |
Certificates of deposit | 14 |  |  | — |  |  | — |  |  | 14 |
Other | 4 |  |  | — |  |  | — |  |  | 4 |
Total current investments | $ | 1,692 |  |  | $ | — |  |  | $ | 11 |  |  | $ | 1,681 |
| December 31, 2017
| Amortized Cost |  | Gross Unrealized |  | Estimated Fair Value
|  | Gains |  | Losses |
| (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 - current restricted investments | 169 |  |  | — |  |  | — |  |  | 169 |
| $ | 2,700 |  |  | $ | 1 |  |  | $ | 8 |  |  | $ | 2,693 |
The contractual maturities of our current investments as of December 31, 2018 are summarized below:
| Amortized Cost |  | Estimated Fair Value
| (In millions)
Due in one year or less | $ | 1,000 |  |  | $ | 997 |
Due after one year through five years | 692 |  |  | 684 |
| $ | 1,692 |  |  | $ | 1,681 |
As discussed further in Note 11, “Debt,” the 4.875% Notes’ indenture required us to hold a portion of the net proceeds from their issuance in a segregated account to be used to settle the conversion of the 1.625% Convertible Notes. Prior to September 30, 2018, this account was reported as a current asset, entitled “Restricted investments,” in the accompanying consolidated balance sheets. Because this account was used to settle the conversion of the 1.625% Convertible Notes in the third quarter of 2018, current restricted investments, as of December 31, 2018, was reduced to zero. 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, 2018, 2017 and 2016 were insignificant. We have determined that unrealized losses at December 31, 2018 and 2017 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, 2018.
| 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 | $ | 509 |  |  | $ | 3 |  |  | 285 |  |  | $ | 412 |  |  | $ | 5 |  |  | 298 |
GSEs | — |  |  | — |  |  | — |  |  | 127 |  |  | 1 |  |  | 76 |
Municipal securities | — |  |  | — |  |  | — |  |  | 87 |  |  | 1 |  |  | 90 |
Asset backed securities | — |  |  | — |  |  | — |  |  | 68 |  |  | 1 |  |  | 52 |
| $ | 509 |  |  | $ | 3 |  |  | 285 |  |  | $ | 694 |  |  | $ | 8 |  |  | 516 |
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 |

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