XOMA Corporation Quarterly report pursuant to Section 13 or 15(d)

Capital Stock - ATM Agreements (Details) - USD ($) $ in Millions						9 Months Ended
	Aug. 05, 2021	Jun. 30, 2021	Apr. 09, 2021	Dec. 15, 2020	Dec. 18, 2018	Sep. 30, 2021
8.625% Series A Cumulative, Perpetual Preferred Stock
Sale of stock
Shares issued (in shares)				984,000
8.375% Series B Cumulative, Perpetual Preferred Stock
Sale of stock
Depositary share's fractional interest in share of preferred stock			0.001			0.001
Depositary Shares
Sale of stock
Shares issued (in shares)			1,600,000
2018 Common Stock ATM Agreement
Sale of stock
Maximum amount of common shares to be issued		$ 50.0			$ 30.0
2018 Common Stock ATM Agreement \| Maximum [Member]
Sale of stock
Sales commission paid per transaction (as a percent)					3.00%
2021 Series B Preferred Stock ATM Agreement
Sale of stock
Maximum amount of shares to be issued	$ 50.0
2021 Series B Preferred Stock ATM Agreement \| 8.375% Series B Cumulative, Perpetual Preferred Stock
Sale of stock
Depositary share's fractional interest in share of preferred stock	0.001
2021 Series B Preferred Stock ATM Agreement \| Depositary Shares
Sale of stock
Shares issued (in shares)						0
2021 Series B Preferred Stock ATM Agreement \| Maximum [Member]
Sale of stock
Sales commission paid per transaction (as a percent)	3.00%
Common Stock \| 2018 Common Stock ATM Agreement
Sale of stock
Shares issued (in shares)		0			0