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Income Taxes (Details Textual) (USD $)
 12 Months Ended
Dec. 31, 2014
Dec. 31, 2013
Dec. 31, 2012
Operating Loss Carryforwards [Line Items]      
Valuation allowance, change in amount $ 9,800,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount $ (5,400,000)us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount $ 41,800,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
Unrecognized tax benefits that would impact deferred tax assets and valuation allowance, and not impact effective tax rate 8,500,000lgnd_UnrecognizedTaxBenefitsThatWouldImpactDeferredTaxAssetsAndValuationAllowanceAndNotImpactEffectiveTaxRate    
Liability for uncertain tax positions 32,000lgnd_LiabilityforUncertainTaxPositions 31,000lgnd_LiabilityforUncertainTaxPositions  
Change to valuation allowance 1,100,000lgnd_ValuationAllowanceUncertainTaxPositionsPeriodChange 0lgnd_ValuationAllowanceUncertainTaxPositionsPeriodChange 0lgnd_ValuationAllowanceUncertainTaxPositionsPeriodChange
Realization Of Excess Tax Benefits From Share Based Compensation      
Operating Loss Carryforwards [Line Items]      
Deferred tax assets not recognized related to share-based compensation 8,100,000lgnd_DeferredTaxAssetsExcessTaxBenefitsFromShareBasedCompensationAmountNotRealized
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= lgnd_RealizationOfExcessTaxBenefitsFromShareBasedCompensationMember		 2,400,000lgnd_DeferredTaxAssetsExcessTaxBenefitsFromShareBasedCompensationAmountNotRealized
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= lgnd_RealizationOfExcessTaxBenefitsFromShareBasedCompensationMember		  
Valuation allowance, change in amount 700,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= lgnd_RealizationOfExcessTaxBenefitsFromShareBasedCompensationMember		 1,000,000us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= lgnd_RealizationOfExcessTaxBenefitsFromShareBasedCompensationMember		 0us-gaap_ValuationAllowanceDeferredTaxAssetChangeInAmount
/ us-gaap_ValuationAllowanceByDeferredTaxAssetAxis
= lgnd_RealizationOfExcessTaxBenefitsFromShareBasedCompensationMember
Research Tax Credit Carryforward      
Operating Loss Carryforwards [Line Items]      
Tax credit carryforward 18,700,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_TaxCreditCarryforwardAxis
= us-gaap_ResearchMember		    
California and New Jersey Research Tax Credit Carryforward      
Operating Loss Carryforwards [Line Items]      
Tax credit carryforward 14,000,000us-gaap_TaxCreditCarryforwardAmount
/ us-gaap_TaxCreditCarryforwardAxis
= lgnd_CaliforniaAndNewJerseyResearchTaxCreditCarryforwardMember		    
Internal Revenue Service (IRS)      
Operating Loss Carryforwards [Line Items]      
Operating loss carryforward 563,700,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_InternalRevenueServiceIRSMember		    
State and Local Jurisdiction      
Operating Loss Carryforwards [Line Items]      
Operating loss carryforward $ 167,300,000us-gaap_OperatingLossCarryforwards
/ us-gaap_IncomeTaxAuthorityAxis
= us-gaap_StateAndLocalJurisdictionMember