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Marketable Securities (Tables) | 3 Months Ended
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Mar. 31, 2018
Marketable Securities [Abstract] |
Amortized Cost And Fair Value Of Marketable Securities |
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﻿As of December 31, 2017 |  |  | Amortized Cost |  |  | Gross Unrealized Gains |  |  | Gross Unrealized Losses |  |  | Fair Value |
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﻿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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