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Marketable Securities - Unrealized losses (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Fair value |  |
Fair value, less than 12 months | $ 268,115 | $ 207,603
Fair value, 12 months or greater | 236,108 | 139,197
Unrealized losses |  |
Unrealized losses, Less than 12 months | (977) | (598)
Unrealized gain (losses), 12 months or greater | 16 | (1,201)
Corporate Securities |  |
Fair value |  |
Fair value, less than 12 months | 84,770 | 79,290
Fair value, 12 months or greater | 20,198 | 43,980
Unrealized losses |  |
Unrealized losses, Less than 12 months | (310) | (137)
Unrealized gain (losses), 12 months or greater | (34) | (453)
Government Securities |  |
Fair value |  |
Fair value, less than 12 months | 183,345 | 128,313
Fair value, 12 months or greater | 215,910 | 95,217
Unrealized losses |  |
Unrealized losses, Less than 12 months | (667) | (461)
Unrealized gain (losses), 12 months or greater | $ 50 | $ (748)

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- Definition Amount of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale), in continuous unrealized loss position for more than 12 months. + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 + Details
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