| Cover Page | Oct. 01, 2020 |
| Cover [Abstract] | |
| Entity Central Index Key | 0000886163 |
| Amendment Flag | false |
| Document Type | 8-K/A |
| Document Period End Date | Oct. 1, 2020 |
| Entity Registrant Name | LIGAND PHARMACEUTICALS INC |
| Entity Incorporation, State or Country Code | DE |
| Entity File Number | 001-33093 |
| Entity Tax Identification Number | 77-0160744 |
| Entity Address, Address Line One | 3911 Sorrento Valley Boulevard, Suite 110 |
| Entity Address, City or Town | San Diego |
| Entity Address, State or Province | CA |
| Entity Address, Postal Zip Code | 92121 |
| City Area Code | 858 |
| Local Phone Number | 550-7500 |
| Written Communications | false |
| Soliciting Material | false |
| Pre-commencement Tender Offer | false |
| Pre-commencement Issuer Tender Offer | false |
| Title of 12(b) Security | Common Stock, par value $0.001 per share |
| Trading Symbol | LGND |
| Security Exchange Name | NASDAQ |
| Entity Emerging Growth Company | false |
