Agenus Transaction	9 Months Ended
Agenus Transaction	Sep. 30, 2024
Business Combination, Asset Acquisition, and Joint Venture Formation [Abstract]
Agenus Transaction	Agenus Transaction On May 29, 2024, we closed the transactions pursuant to the $75 million purchase and sale agreement (the “Agenus Agreement”), dated May 6, 2024, among us and Agenus Inc., Agenus Royalty Fund, LLC, and Agenus Holdings 2024, LLC (collectively, “Agenus”). Under the terms of the Agenus Agreement, we received (i) 18.75% of the licensed royalties and 31.875% of the future licensed milestones paid to Agenus on six-partnered oncology programs, including BMS-986442 (Bristol Myers Squibb), AGEN2373 (Gilead Sciences), INCAGN2385 and INCAGN2390 (Incyte), MK-4830 (Merck), and UGN-301 (UroGen Pharma) (collectively referred as “Agenus Partnered Programs”), and (ii) a synthetic 2.625% royalty on future global net sales of Agenus’ novel immuno-oncology botensilimab in combination with balstilimab (“BOT/BAL”) program, collectively subject to certain events which may adjust the royalty and milestone percentages paid to us. In addition, we received the option to commit an additional $25 million in the same assets on a pro rata basis which expires on June 30, 2025 (“Upsize Option”). We have also agreed to allow Agenus to raise up to an additional $100 million bringing the total syndicated purchase price up to an aggregate of $200 million. As part of the Agenus Agreement, Agenus will grant us security over certain assets related to the programs included in the Agenus Agreement, subject to certain customary exceptions. In connection with entry into the Agenus Agreement, Agenus issued us a 5-year warrant (“Agenus Warrant”) to purchase 867,052 shares of its common stock, at an exercise price equal to $17.30. We accounted for all Agenus Partnered Programs, Agenus Warrant and Upsize Option as derivative assets. Upsize Option was presented within current derivative assets line (as it expires on June 30, 2025), and the other derivatives were presented in noncurrent derivative assets line in our condensed consolidated balance sheet. Agenus Partnered Programs are recognized as derivative assets under ASC 815, Derivatives and Hedging, as they have different underlyings (milestone payments and royalties). The commercial milestones and royalties are dependent on the development milestones and the commercial milestone and royalties underlyings are not determined to be predominate. The derivative assets were recorded at fair value as of May 29, 2024, and are marked to fair value at each subsequent reporting period. The fair value of Agenus Partnered Programs derivative assets is determined as a present value of expected future cash flows adjusted for the level of risk appropriate for a respective program stage. During the three months ended September 30, 2024, certain Agenus partners discontinued development of their partnered programs. These programs may be relicensed at a later date, and Ligand would retain its economic interest upon any relicense activity. The fair value of Agenus Warrant is determined using a Black-Scholes model. The following assumptions were used as of May 29, 2024, and September 30, 2024, respectively: expected term of 4.0 years and 3.7 years, volatility of 84% and 99%, risk-free rate of 4.7% and 3.6%, Agenus Stock price of $15.03 and $5.48. The fair value of the Upsize Option was determined using the binomial option pricing model under which we assessed and considered the possible upwards and downwards scenarios through the expiration date of the Upsize Option. See Note 7, Fair Value Measurements, for additional information on the Agenus Partnered Program derivative assets, Agenus Warrant, and Upsize Option. We accounted for the acquired BOT/BAL rights as a financial royalty asset which is currently put under the non-accrual method as management cannot reliably estimate future cash flows from this program. The amount of BOT/BAL financial royalty asset was determined as a residual value from the $75 million aggregate investment amount, less fair value of all acquired derivative assets as of May 29, 2024.