| Investments |
Investments Our investments consisted of the following (in thousands): | | | | | | | | | | | | | | | | | | | | | | | | | | As of December 31, 2018 | | Amortized Cost | | Gross Unrealized Gains | | Gross Unrealized Losses | | Estimated Fair Value | | Cash and Cash Equivalents | | Short-Term Investments | Corporate notes and bonds | $ | 450,097 |
| | $ | 44 |
| | $ | (1,818 | ) | | $ | 448,323 |
| | $ | — |
| | $ | 448,323 |
| U.S. Treasuries | 112,783 |
| | 2 |
| | (85 | ) | | 112,700 |
| | — |
| | 112,700 |
| U.S. Government agencies | 146,110 |
| | — |
| | (442 | ) | | 145,668 |
| | — |
| | 145,668 |
| Total | $ | 708,990 |
|
| $ | 46 |
|
| $ | (2,345 | ) |
| $ | 706,691 |
|
| $ | — |
| | $ | 706,691 |
|
| | | | | | | | | | | | | | | | | | | | | | | | | | As of December 31, 2017 | | Amortized Cost | | Gross Unrealized Gains | | Gross Unrealized Losses | | Estimated Fair Value | | Cash and Cash Equivalents | | Short-Term Investments | Commercial paper | $ | 4,989 |
| | $ | — |
| | $ | (2 | ) | | $ | 4,987 |
| | $ | — |
| | $ | 4,987 |
| Corporate notes and bonds | 439,851 |
| | 2 |
| | (1,829 | ) | | 438,024 |
| | — |
| | 438,024 |
| U.S. Treasuries | 3,098 |
| | — |
| | — |
| | 3,098 |
| | 3,098 |
| | — |
| U.S. Government agencies | 273,950 |
| | — |
| | (1,050 | ) | | 272,900 |
| | — |
| | 272,900 |
| Total | $ | 721,888 |
|
| $ | 2 |
|
| $ | (2,881 | ) |
| $ | 719,009 |
|
| $ | 3,098 |
| | $ | 715,911 |
|
The following tables present the gross unrealized losses and related fair values of our investments that have been in a continuous unrealized loss position (in thousands): | | | | | | | | | | | | | | | | | | | | | | | | | | As of December 31, 2018 | | Less Than 12 Months | | Greater Than 12 Months | | Total | | Fair Value | | Unrealized Loss | | Fair Value | | Unrealized Loss | | Fair Value | | Unrealized Loss | Corporate notes and bonds | $ | 420,548 |
| | $ | (1,817 | ) | | $ | 1,526 |
| | $ | (2 | ) | | $ | 422,074 |
| | $ | (1,819 | ) | U.S. Treasuries | 105,525 |
| | (85 | ) | | — |
| | — |
| | 105,525 |
| | (85 | ) | U.S. Government agencies | 137,416 |
| | (441 | ) | | — |
| | — |
| | 137,416 |
| | (441 | ) | Total | $ | 663,489 |
|
| $ | (2,343 | ) |
| $ | 1,526 |
|
| $ | (2 | ) |
| $ | 665,015 |
|
| $ | (2,345 | ) |
| | | | | | | | | | | | | | | | | | | | | | | | | | As of December 31, 2017 | | Less Than 12 Months | | Greater Than 12 Months | | Total | | Fair Value | | Unrealized Loss | | Fair Value | | Unrealized Loss | | Fair Value | | Unrealized Loss | Commercial paper | $ | 4,987 |
| | $ | (2 | ) | | $ | — |
| | $ | — |
| | $ | 4,987 |
| | $ | (2 | ) | Corporate notes and bonds | 284,499 |
| | (1,484 | ) | | 153,525 |
| | (345 | ) | | 438,024 |
| | (1,829 | ) | U.S. Treasuries | — |
| | — |
| | — |
| | — |
| | — |
| | — |
| U.S. Government agencies | 117,132 |
| | (486 | ) | | 155,768 |
| | (564 | ) | | 272,900 |
| | (1,050 | ) | Total | $ | 406,618 |
|
| $ | (1,972 | ) |
| $ | 309,293 |
|
| $ | (909 | ) |
| $ | 715,911 |
|
| $ | (2,881 | ) |
Unrealized losses related to these investments are due to interest rate fluctuations as opposed to credit quality. In addition, we do not intend to sell, and it is not more likely than not that we would be required to sell, these investments before recovery of their cost basis. As a result, there is no other-than-temporary impairment for these investments as of December 31, 2018 and 2017. The following table summarizes the contractual maturities of our investments at December 31, 2018 (in thousands): | | | | | | | | | | Amortized Cost | | Fair Value | Due within one year | $ | 460,480 |
| | $ | 458,749 |
| Due within one to three years | 248,510 |
| | 247,942 |
| Total | $ | 708,990 |
|
| $ | 706,691 |
|
All available-for-sale securities have been classified as current, based on management's intent and ability to use the funds in current operations. As of December 31, 2018, we held a 11.1% ownership interest in a privately held company which is accounted for under the equity method based on our ability to exercise significant influence over the company's operating and financial policies. Our investments in this company are classified within deposits and other long-term assets on our consolidated balance sheets. The carrying value of our investments was $0.5 million and $2.1 million as of December 31, 2018 and 2017, respectively.
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