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Short-term Investments | 12 Months Ended
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Dec. 31, 2017
Short-term Investments [Abstract] |
Short-Term Investments | SHORT-TERM INVESTMENTS Short-term investments for the periods presented were comprised of the following (in thousands):
| December 31, 2016
Security Type | Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
U.S. government fixed income securities | $ | 19,403 |  |  | $ | 40 |  |  | $ | — |  |  | $ | 19,443 |
There were no short-term investments at December 31, 2017. Short-term investments at December 31, 2016 were comprised of investments in highly liquid, AAA, U.S. government fixed income securities with maturity dates in 2017. For the years ended December 31, 2017 and 2016, proceeds from the sale of short-term investments classified as available-for-sale were $467,790,000 and $43,232,000, respectively. Gross unrealized gains and losses were not material for the years ended December 31, 2017 and 2016. For the year ended December 31, 2015, proceeds from the sale of short-term investments were $82,115,000 and gross realized losses were $617,000.

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