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Marketable Securities (Tables) | 9 Months Ended
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Sep. 30, 2018
Marketable Securities [Abstract] |
Amortized cost and fair value of marketable securities | The amortized cost and fair value of marketable securities as of December 31, 2017, were as follows (in thousands): ໿
As of December 31, 2017 |  | Amortized Cost |  | Gross Unrealized Gains |  | Gross Unrealized Losses |  | Fair Value
﻿ |  |  |  |  |  |  |  |  |  |  |
Corporate bonds |  | $ | 140,969 |  |  | $ | 96 |  |  | $ | (179 | ) |  | $ | 140,886 |
Certificates of deposit |  | 58,510 |  |  | — |  |  | — |  |  | 58,510 |
Commercial paper |  | 29,171 |  |  | — |  |  | — |  |  | 29,171 |
Asset backed securities |  | 22,206 |  |  | 4 |  |  | (43 | ) |  | 22,167 |
U.S. government bonds |  | 15,619 |  |  | 11 |  |  | (19 | ) |  | 15,611 |
Agency bonds |  | 10,990 |  |  | 9 |  |  | (52 | ) |  | 10,947 |
Treasury bills |  | 6,964 |  |  | — |  |  | (1 | ) |  | 6,963 |
Total marketable securities |  | $ | 284,429 |  |  | $ | 120 |  |  | $ | (294 | ) |  | $ | 284,255 |

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- Definition Tabular disclosure of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -URI http://asc.fasb.org/topic&trid=2196928 + Details
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