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Marketable Securities (Tables) | 6 Months Ended
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Jun. 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 the reconciliation of available-for-sale securities from cost basis to fair value. + References No definition available. + Details
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