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Fair value of financial instruments and investments - Marketable Securities, Unrealized Gains (Losses) (Details) - USD ($) $ in Thousands | Dec. 31, 2018 | Dec. 31, 2017
--- | --- | ---
Marketable securities accounted for as available-for-sale securities |  |
Amortized Cost | $ 58,056 | $ 79,432
Gross Unrealized Gains | 43 | 52
Gross Unrealized Losses | (11) | (30)
Fair Value | 58,088 | 79,454
Commercial paper |  |
Marketable securities accounted for as available-for-sale securities |  |
Amortized Cost | 31,657 | 13,775
Gross Unrealized Gains | 43 | 52
Gross Unrealized Losses | (1) | 0
Fair Value | 31,699 | 13,827
Corporate debt securities |  |
Marketable securities accounted for as available-for-sale securities |  |
Amortized Cost | 26,399 | 65,657
Gross Unrealized Gains | 0 | 0
Gross Unrealized Losses | (10) | (30)
Fair Value | $ 26,389 | $ 65,627

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- Definition Amount, before tax, of unrealized loss in accumulated other comprehensive income (AOCI) on investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (c) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amortized cost of investment in debt security measured at fair value with change in fair value recognized in other comprehensive income (available-for-sale). + References Reference 1: http://www.xbrl.org/2003/role/disclosureRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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- Definition Amount before tax of unrealized gain in accumulated other comprehensive income (AOCI) on investments in debt and equity securities classified as available-for-sale. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 320 -SubTopic 10 -Section 50 -Paragraph 2 -Subparagraph (b) -URI http://asc.fasb.org/extlink&oid=82897076&loc=d3e27161-111563 + Details
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