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Marketable Securities - Continuous Unrealized Loss Position (Details) $ in Thousands | Dec. 31, 2017 USD ($)
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Investments in an Unrealized Loss Position |  |
Fair Value | $ 1,542,909 |
Gross Unrealized Losses, less than 12 months | (9,866) | [1]
Gross Unrealized Losses, 12 months or more | 5,300 |
Corporate debt securities |  |
Investments in an Unrealized Loss Position |  |
Fair Value | 792,436 |
Gross Unrealized Losses, less than 12 months | (4,245) | [1]
U.S. Treasury securities |  |
Investments in an Unrealized Loss Position |  |
Fair Value | 404,982 |
Gross Unrealized Losses, less than 12 months | (2,941) | [1]
U.S. Government agency securities |  |
Investments in an Unrealized Loss Position |  |
Fair Value | 334,064 |
Gross Unrealized Losses, less than 12 months | (2,611) | [1]
Sovereign securities |  |
Investments in an Unrealized Loss Position |  |
Fair Value | 11,427 |
Gross Unrealized Losses, less than 12 months | $ (69) | [1]
[1] | As of December 31, 2017, the amount of total gross unrealized losses related to investments that had been in a continuous loss position for 12 months or more was $5.3 million.

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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous 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 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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- Definition Amount of accumulated unrealized loss on investments in debt and equity securities classified as available-for-sale that have been in a continuous 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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- Definition This item represents the aggregate fair value of investments in debt and equity securities categorized neither as held-to-maturity nor trading securities that have been in a continuous unrealized 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 7 -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27337-111563 Reference 2: http://www.xbrl.org/2003/role/presentationRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 6 -Subparagraph (a)(1) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27290-111563 + Details
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