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Marketable Securities (Tables) | 12 Months Ended
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Dec. 31, 2017
Marketable Securities |
Summary of marketable securities, classified as available-for-sale |
|  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value |
|  | (In thousands) |
December 31, 2017 |  |  |  |  |  |  |  |  |  |  |  |  |
Marketable securities |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. government and municipal obligations |  |  |  |  |  |  |  |  |  |  |  |  |
Maturing in one year or less |  | $ | 26,164 |  | $ | 3 |  | $ | (9 | ) | $ | 26,158 |
Maturing after one year through three years |  |  | — |  |  | — |  |  | — |  |  | — |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Total U.S. government and municipal obligations |  | $ | 26,164 |  | $ | 3 |  | $ | (9 | ) | $ | 26,158 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Corporate debt securities |  |  |  |  |  |  |  |  |  |  |  |  |
Maturing in one year or less |  | $ | 73,007 |  | $ | 1 |  | $ | (27 | ) | $ | 72,981 |
Maturing after one year through three years |  |  | — |  |  | — |  |  | — |  |  | — |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Total corporate debt securities |  | $ | 73,007 |  | $ | 1 |  | $ | (27 | ) | $ | 72,981 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Total marketable securities |  | $ | 99,171 |  | $ | 4 |  | $ | (36 | ) | $ | 99,139 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
December 31, 2016 |  |  |  |  |  |  |  |  |  |  |  |  |
Marketable securities |  |  |  |  |  |  |  |  |  |  |  |  |
U.S. government and municipal obligations |  |  |  |  |  |  |  |  |  |  |  |  |
Maturing in one year or less |  | $ | 52,754 |  | $ | 5 |  | $ | (12 | ) | $ | 52,747 |
Maturing after one year through three years |  |  | 296 |  |  | 8 |  |  | — |  |  | 304 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Total U.S. government and municipal obligations |  | $ | 53,050 |  | $ | 13 |  | $ | (12 | ) | $ | 53,051 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Corporate debt securities |  |  |  |  |  |  |  |  |  |  |  |  |
Maturing in one year or less |  | $ | 94,320 |  | $ | — |  | $ | (56 | ) | $ | 94,264 |
Maturing after one year through three years |  |  | — |  |  | — |  |  | — |  |  | — |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Total corporate debt securities |  | $ | 94,320 |  | $ | — |  | $ | (56 | ) | $ | 94,264 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
Total marketable securities |  | $ | 147,370 |  | $ | 13 |  | $ | (68 | ) | $ | 147,315 |
​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​ | ​
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