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

Fair Value Measurements - Estimated Fair Value of Equity Securities Assumptions (Details)		9 Months Ended
	Apr. 03, 2018	Sep. 30, 2018
Discount for Lack of Marketability [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Valuation assumptions, measurement input	30	35
Liquidity of Shares [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Valuation assumptions, measurement input term	1 year 5 months 12 days	1 year 5 months 12 days
Liquidation [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Valuation assumptions, measurement input	65	80
Near-term Sale [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Valuation assumptions, measurement input	5	5
Near-term Financing [Member]
Fair Value Assets And Liabilities Measured On Recurring And Nonrecurring Basis Valuation Techniques [Line Items]
Valuation assumptions, measurement input	30	15