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INVESTMENTS (Tables) | 12 Months Ended
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Dec. 31, 2017
Investments Schedule [Abstract] |
Investments | Investments at December 31, 2017 and 2016 consisted of the following (in thousands):
|  | Years Ended December 31, |
|  | 2017 |  |  | 2016 |
Certificates of deposit |  | $ | — |  |  | $ | 960 |
Commercial paper |  |  | 75,362 |  |  |  | 49,245 |
Corporate debt securities |  |  | 414,815 |  |  |  | 204,436 |
Securities of government-sponsored entities |  |  | 18,401 |  |  |  | 12,932 |
Total investments |  | $ | 508,578 |  |  | $ | 267,573 |
Summary of Investments Classified as Available-For-Sale Securities | The following is a summary of investments classified as available-for-sale securities (in thousands):
|  | Contractual Maturity (in years) |  |  | Amortized Cost |  |  | Gross Unrealized Gains(1) |  |  | Gross Unrealized Losses(1) |  |  | Aggregate Estimated Fair Value |
December 31, 2016: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Classified as current assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Certificates of deposit |  |  | Less than 1 |  |  | $ | 960 |  |  | $ | — |  |  | $ | — |  |  | $ | 960 |
Commercial paper |  |  | Less than 1 |  |  |  | 49,280 |  |  |  | 3 |  |  |  | (38 | ) |  |  | 49,245 |
Corporate debt securities |  |  | Less than 1 |  |  |  | 168,548 |  |  |  | 3 |  |  |  | (117 | ) |  |  | 168,434 |
Securities of government-sponsored entities |  |  | Less than 1 |  |  |  | 5,448 |  |  |  | — |  |  |  | (4 | ) |  |  | 5,444 |
Total short-term available-for-sale securities |  |  |  |  |  | $ | 224,236 |  |  | $ | 6 |  |  | $ | (159 | ) |  | $ | 224,083 |
Classified as non-current assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate debt securities |  |  | 1 to 2 |  |  | $ | 36,149 |  |  | $ | — |  |  | $ | (147 | ) |  | $ | 36,002 |
Securities of government-sponsored entities |  |  | 1 to 2 |  |  |  | 7,506 |  |  |  | — |  |  |  | (18 | ) |  |  | 7,488 |
Total long-term available-for-sale securities |  |  |  |  |  | $ | 43,655 |  |  | $ | — |  |  | $ | (165 | ) |  | $ | 43,490 |
December 31, 2017: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Classified as current assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  |  | Less than 1 |  |  | $ | 75,396 |  |  | $ | 1 |  |  | $ | (35 | ) |  | $ | 75,362 |
Corporate debt securities |  |  | Less than 1 |  |  |  | 178,776 |  |  |  | — |  |  |  | (400 | ) |  |  | 178,376 |
Securities of government-sponsored entities |  |  | Less than 1 |  |  |  | 7,503 |  |  |  | — |  |  |  | (24 | ) |  |  | 7,479 |
Total short-term available-for-sale securities |  |  |  |  |  | $ | 261,675 |  |  | $ | 1 |  |  | $ | (459 | ) |  | $ | 261,217 |
Classified as non-current assets: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Corporate debt securities |  |  | 1 to 2 |  |  | $ | 237,749 |  |  | $ | — |  |  | $ | (1,310 | ) |  | $ | 236,439 |
Securities of government-sponsored entities |  |  | 1 to 2 |  |  |  | 11,004 |  |  |  | — |  |  |  | (82 | ) |  |  | 10,922 |
Total long-term available-for-sale securities |  |  |  |  |  | $ | 248,753 |  |  | $ | — |  |  | $ | (1,392 | ) |  | $ | 247,361 |
(1) | Unrealized gains and losses are included in other comprehensive loss.
Gross Unrealized Losses and Fair Value Available-For-Sale Investments in Unrealized Loss Position | The following table presents gross unrealized losses and fair value for those available-for-sale investments that were in an unrealized loss position as of December 31, 2017 and 2016, aggregated by investment category and length of time that individual securities have been in a continuous loss position (in thousands):
|  | Less Than 12 Months |  |  | 12 Months or Greater |  |  | Total |
|  | Estimated Fair Value |  |  | Unrealized Losses |  |  | Estimated Fair Value |  |  | Unrealized Losses |  |  | Estimated Fair Value |  |  | Unrealized Losses |
December 31, 2016: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 43,781 |  |  | $ | (38 | ) |  | $ | — |  |  | $ | — |  |  | $ | 43,781 |  |  | $ | (38 | )
Corporate debt securities |  |  | 185,243 |  |  |  | (261 | ) |  |  | 9,144 |  |  |  | (3 | ) |  |  | 194,387 |  |  |  | (264 | )
Securities of government-sponsored entities |  |  | 12,932 |  |  |  | (22 | ) |  |  | — |  |  |  | — |  |  |  | 12,932 |  |  |  | (22 | )
Total |  | $ | 241,956 |  |  | $ | (321 | ) |  | $ | 9,144 |  |  | $ | (3 | ) |  | $ | 251,100 |  |  | $ | (324 | )
December 31, 2017: |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Commercial paper |  | $ | 62,602 |  |  | $ | (35 | ) |  | $ | — |  |  | $ | — |  |  | $ | 62,602 |  |  | $ | (35 | )
Corporate debt securities |  |  | 386,728 |  |  |  | (1,660 | ) |  |  | 28,087 |  |  |  | (50 | ) |  |  | 414,815 |  |  |  | (1,710 | )
Securities of government-sponsored entities |  |  | 10,922 |  |  |  | (82 | ) |  |  | 7,479 |  |  |  | (24 | ) |  |  | 18,401 |  |  |  | (106 | )
Total |  | $ | 460,252 |  |  | $ | (1,777 | ) |  | $ | 35,566 |  |  | $ | (74 | ) |  | $ | 495,818 |  |  | $ | (1,851 | )

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