Acquisitions - Intangibles Acquired and Weighted Average Useful Life (Details) - Pfenex $ in Thousands	Oct. 01, 2020 USD ($)
Business Acquisition [Line Items]
Approximate Fair Value	$ 385,000
Contractual Relationships - Alvogen
Business Acquisition [Line Items]
Approximate Fair Value	$ 114,000
Estimated useful life (in years)	12 years
Contractual Relationships - Merck
Business Acquisition [Line Items]
Approximate Fair Value	$ 117,000
Estimated useful life (in years)	12 years
Contractual Relationships - Jazz
Business Acquisition [Line Items]
Approximate Fair Value	$ 80,000
Estimated useful life (in years)	17 years
Contractual Relationships - SII
Business Acquisition [Line Items]
Approximate Fair Value	$ 49,000
Estimated useful life (in years)	10 years
Contractual Relationships - Arcellx
Business Acquisition [Line Items]
Approximate Fair Value	$ 2,000
Estimated useful life (in years)	17 years
Acquired Technologies
Business Acquisition [Line Items]
Approximate Fair Value	$ 23,000
Minimum \| Acquired Technologies
Business Acquisition [Line Items]
Estimated useful life (in years)	10 years
Maximum \| Acquired Technologies
Business Acquisition [Line Items]
Estimated useful life (in years)	19 years

Weighted average amortization period of finite-lived intangible assets acquired either individually or as part of a group of assets, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 350
-SubTopic 30
-Section 50
-Paragraph 1
-Subparagraph (a)(3)
-URI http://asc.fasb.org/extlink&oid=66006027&loc=d3e16265-109275

+ Details

Name:	us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife
Namespace Prefix:	us-gaap_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_BusinessAcquisitionLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of intangible assets, excluding goodwill, acquired at the acquisition date.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 805
-SubTopic 20
-Section 50
-Paragraph 1
-Subparagraph (c)
-URI http://asc.fasb.org/extlink&oid=123413009&loc=d3e4845-128472

+ Details

Name:	us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Details

Name:	us-gaap_BusinessAcquisitionAxis=lgnd_PfenexMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=lgnd_ContractualRelationshipsAlvogenMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=lgnd_ContractualRelationshipsMerckMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=lgnd_ContractualRelationshipsJazzMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=lgnd_ContractualRelationshipsSIIMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=lgnd_ContractualRelationshipsArcellxMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis=us-gaap_TechnologyBasedIntangibleAssetsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: