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Fair value of financial instruments and marketable securities - Unrealized Loss Positions (Details) - USD ($) $ in Thousands | Mar. 31, 2019 | Dec. 31, 2018
--- | --- | ---
Unrealized losses |  |
Securities in an unrealized loss position less than 12 months | $ (47) | $ (8)
Securities in an unrealized loss position greater than 12 months | 0 | (3)
Total | (47) | (11)
Fair Value |  |
Securities in an unrealized loss position less than 12 months | 67,793 | 16,223
Securities in an unrealized loss position greater than 12 months | 0 | 10,087
Total | 67,793 | 26,310
Commercial paper |  |
Unrealized losses |  |
Securities in an unrealized loss position less than 12 months | 0 | (1)
Securities in an unrealized loss position greater than 12 months | 0 | 0
Total | 0 | (1)
Fair Value |  |
Securities in an unrealized loss position less than 12 months | 0 | 1,993
Securities in an unrealized loss position greater than 12 months | 0 | 0
Total | 0 | 1,993
Corporate debt securities |  |
Unrealized losses |  |
Securities in an unrealized loss position less than 12 months | (46) | (7)
Securities in an unrealized loss position greater than 12 months | 0 | (3)
Total | (46) | (10)
Fair Value |  |
Securities in an unrealized loss position less than 12 months | 61,716 | 14,230
Securities in an unrealized loss position greater than 12 months | 0 | 10,087
Total | 61,716 | $ 24,317
Asset-backed securities |  |
Unrealized losses |  |
Securities in an unrealized loss position less than 12 months | (1) |
Securities in an unrealized loss position greater than 12 months | 0 |
Total | (1) |
Fair Value |  |
Securities in an unrealized loss position less than 12 months | 6,077 |
Securities in an unrealized loss position greater than 12 months | 0 |
Total | $ 6,077 |

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- Definition Amount of accumulated unrealized loss on 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 12 months or longer. + 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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- Definition Amount of accumulated unrealized loss on 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 less 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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