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Marketable Securities - Unrealized losses (Details) - USD ($) $ in Thousands | 12 Months Ended
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Dec. 31, 2017 | Dec. 31, 2016
Fair value |  |
Fair value, less than 12 months | $ 207,603 | $ 99,788
Fair value, 12 months or greater | 139,197 | 238,684
Unrealized losses |  |
Unrealized losses, Less than 12 months | (598) | (149)
Unrealized losses, 12 months or greater | (1,201) | (1,333)
Corporate Securities |  |
Fair value |  |
Fair value, less than 12 months | 79,290 | 82,215
Fair value, 12 months or greater | 43,980 | 88,990
Unrealized losses |  |
Unrealized losses, Less than 12 months | (137) | (133)
Unrealized losses, 12 months or greater | (453) | (784)
Government Securities |  |
Fair value |  |
Fair value, less than 12 months | 128,313 | 17,573
Fair value, 12 months or greater | 95,217 | 149,694
Unrealized losses |  |
Unrealized losses, Less than 12 months | (461) | (16)
Unrealized losses, 12 months or greater | $ (748) | $ (549)

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- Definition Amount of unrealized loss on securities classified as available-for-sale that have been in a loss position for twelve months or longer. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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- Definition Amount of unrealized loss on investments in available-for-sale securities that have been in a loss position for less than twelve months. + References Reference 1: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(2) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -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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