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Fair Value Measurements - Narrative (Details) - USD ($) $ in Millions | 3 Months Ended
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Sep. 30, 2019 | Mar. 31, 2019 | Mar. 31, 2018
Fair Value Disclosures [Abstract] |  |  |
Increase (decrease) in contingent liability |  | $ (1.5) |
Repayments of related party debt | $ 3.0 |  | $ 1.0

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- References No definition available. + Details
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Namespace Prefix: | lgnd_
Data Type: | xbrli:monetaryItemType
Balance Type: | credit
Period Type: | duration

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- References No definition available. + Details
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- Definition Amount of increase (decrease) in obligation to transfer good or service to customer for which consideration has been received or is receivable. + References Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef -Publisher FASB -Name Accounting Standards Codification -Topic 230 -SubTopic 10 -Section 45 -Paragraph 28 -Subparagraph (a) -URI http://asc.fasb.org/extlink&oid=118932676&loc=d3e3602-108585 + Details
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