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Short-term and Long-term Investments | 12 Months Ended
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Dec. 31, 2016
Investments, Debt and Equity Securities [Abstract] |
Short-term and Long-term Investments | Short-term and Long-term Investments The Company's investments are classified as available-for-sale. The following table summarizes the amortized cost, gross unrealized gains and losses and fair value of available-for-sale investments as of December 31, 2016:
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Aggregate Fair Value
U.S. government debt securities | $ | 180,412 |  |  | $ | 5 |  |  | $ | (94 | ) |  | $ | 180,323 |
Certificates of deposit | 272 |  |  | — |  |  | — |  |  | 272 |
Total | $ | 180,684 |  |  | $ | 5 |  |  | $ | (94 | ) |  | $ | 180,595 |
The following table summarizes the amortized cost, gross unrealized gains and losses and fair value of available-for-sale investments as of December 31, 2015:
| Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Aggregate Fair Value
U.S. government debt securities | $ | 208,223 |  |  | $ | 21 |  |  | $ | (540 | ) |  | $ | 207,704 |
Certificates of deposit | 271 |  |  | — |  |  | — |  |  | 271 |
Total | $ | 208,494 |  |  | $ | 21 |  |  | $ | (540 | ) |  | $ | 207,975 |
For more information on the Company's method for determining the fair value of its assets, see Note 2 – "Fair Value of Financial Instruments". The estimated fair value of available-for-sale investments classified by their contractual maturities as of December 31, 2016 was:
Due within one year | $ | 174,602 |
After one year through two years | 5,993 |
Total | $ | 180,595 |
Changes in market interest rates and bond yields cause certain investments to fall below their cost basis, resulting in unrealized losses on investments. The unrealized losses of the Company's investments were primarily a result of unfavorable changes in interest rates subsequent to the initial purchase of these investments and have been in a loss position for less than 12 months. As of December 31, 2016 and 2015, the Company did not consider any of its investments to be other-than-temporarily impaired. When evaluating its investments for other-than-temporary impairment, the Company reviews factors such as the length of time and extent to which fair value has been below its cost basis, the financial condition of the issuer, the Company's ability and intent to hold the security and whether it is more likely than not that it will be required to sell the investment before recovery of its cost basis.

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