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Fair Value Measurements - Additional information (Details) - USD ($) $ in Thousands | 12 Months Ended
--- | ---
Dec. 31, 2018 | Dec. 31, 2017
Fair value measurements |  |
Gain (loss) on fair value remeasurement of contingent consideration | $ 29,621 | $ 800
Kolltan | Specified development, regulatory approvals or sales-based milestones |  |
Fair value measurements |  |
Milestone payments | 127,500 |
Fair value |  |
Fair value measurements |  |
Assets level 1 to level 2 transfers | 0 | 0
Assets level 2 to level 1 transfers | 0 | 0
Liabilities level 1 to level 2 transfers | 0 | 0
Liabilities level 2 to level 1 transfers | 0 | 0
Asset transfer out of level 3 | 0 | 0
Asset transfer into level 3 | 0 | 0
Liabilities transfers out of level 3 | 0 | 0
Liabilities transfers into level 3 | $ 0 | $ 0

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- Definition For contingent consideration arrangements recognized in connection with a business combination, this element represents an estimate of the high-end of the potential range (undiscounted) of the consideration which may be paid. + References Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef -Publisher FASB -Name Accounting Standards Codification -Topic 805 -SubTopic 30 -Section 50 -Paragraph 1 -Subparagraph (c)(3) -URI http://asc.fasb.org/extlink&oid=35744584&loc=d3e6927-128479 + Details
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