XOMA Corporation Annual report pursuant to Section 13 and 15(d)

Fair Value Measurements - Estimated Fair Value of Equity Securities Assumptions (Details)	10 Months Ended	12 Months Ended
	Dec. 31, 2018 shares	Dec. 31, 2020 Y $ / shares item shares	Dec. 31, 2019 $ / shares Y shares
License Agreement and Common Stock Purchase Agreement \| Rezolute
Fair Value Measurements
Number of Rezolute's shares Company has right and option to sell if Rezolute does not list common stock shares on national exchange \| shares		100,000
Number of tranches \| item		2
Common Stock Purchase Agreement \| Rezolute
Fair Value Measurements
Number of Rezolute's shares Company has right and option to sell if Rezolute does not list common stock shares on national exchange \| shares	5,000,000	2,500,000	5,000,000
Closing common stock price
Fair Value Measurements
Valuation assumptions, measurement input \| $ / shares		11.99	6.00
Closing common stock price \| Previously Reported
Fair Value Measurements
Valuation assumptions, measurement input \| $ / shares			0.12
Discount for lack of marketability \| Tranche 1
Fair Value Measurements
Valuation assumptions, measurement input		0.12	0.13
Discount for lack of marketability \| Tranche 2
Fair Value Measurements
Valuation assumptions, measurement input		0.14	0.33
Estimated time to liquidity of shares \| Tranche 1
Fair Value Measurements
Valuation assumptions, measurement input \| Y		0.25	0.25
Estimated time to liquidity of shares \| Tranche 2
Fair Value Measurements
Valuation assumptions, measurement input \| Y		0.67	1.5