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Licensing and Other Arrangements - Rezolute - Additional Information (Details) |  |  |  |  |  |  |  | 1 Months Ended | 2 Months Ended | 3 Months Ended | 9 Months Ended | 12 Months Ended | 18 Months Ended | 21 Months Ended |
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Mar. 31, 2020 USD ($) | Sep. 30, 2019 USD ($) | Mar. 31, 2019 USD ($) | Jan. 30, 2019 USD ($) | Jan. 07, 2019 USD ($) CashPayment | Apr. 03, 2018 USD ($) Obligation shares | Dec. 06, 2017 USD ($) shares | Feb. 28, 2019 USD ($) | Apr. 30, 2018 USD ($) shares | Mar. 31, 2018 USD ($) shares | Aug. 31, 2019 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Mar. 31, 2018 USD ($) | Sep. 30, 2019 USD ($) | Sep. 30, 2018 USD ($) | Dec. 31, 2020 shares | Dec. 31, 2018 USD ($) | Jun. 01, 2019 USD ($) | Sep. 30, 2020 USD ($) | Jun. 30, 2019 USD ($)
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Revenue from contracts with customers |  |  |  |  |  |  |  |  |  |  |  | $ 8,525,000 | $ 775,000 |  | $ 17,176,000 | $ 3,518,000 |  |  |  |  |
Contract revenue |  |  |  |  |  |  |  |  |  |  |  | 8,525,000 | $ 775,000 |  | 17,176,000 | $ 3,518,000 |  |  |  |  |
Rezolute [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Contract revenue |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 0 |  |  |  |  |  |  |
Rezolute [Member] | License Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Future milestone payments, maximum |  |  |  |  |  |  | $ 232,000,000 |  |  |  |  |  |  |  |  |  |  |  | $ 237,000,000 |  |
Percentage of decrease in future royalty obligations |  |  |  |  |  |  | 20.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Upfront option fee |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,000,000 |  |
Agreement termination notice period |  |  |  |  |  |  | 90 days |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reimbursable technology transfer expenses received |  |  |  |  |  | $ 300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock value receivable |  |  |  |  | $ 8,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of future cash payments upon closing of the qualified financing | CashPayment |  |  |  |  | 5 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Percentage of net proceeds of each future financing upon closing of the qualified financing |  |  |  |  | 15.00% |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Doubtful revenue reversal of future cash payments |  |  |  |  |  |  |  |  |  |  | $ 3,600,000 |  |  |  |  |  |  |  |  |  |
Early payment of unrecognized future cash payments |  |  |  |  |  |  |  |  |  |  | 3,400,000 |  |  |  |  |  |  |  |  |  |
Unrecognized future cash payments |  |  |  |  |  |  |  |  |  |  | $ 6,000,000 |  |  |  |  |  |  |  |  |  |
Future cash payments received |  | $ 1,500 |  |  |  |  |  |  |  |  |  | 4,900 |  |  |  |  |  |  |  |  |
Revenue from contracts with customers |  |  |  |  |  |  |  |  |  |  |  | 6,000,000 |  |  |  |  |  |  |  |  |
Outstanding receivable |  | 3,600,000 |  |  |  |  |  |  |  |  |  | 3,600,000 |  |  | 3,600,000 |  |  |  |  |  |
Contract revenue |  |  |  |  |  |  |  |  |  |  |  | 6,000,000 |  |  |  |  |  |  |  |  |
Rezolute [Member] | License Agreement [Member] | Forecast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Future cash payments upon closing of the qualified financing |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,500,000 |
Rezolute [Member] | License Agreement and Common Stock Purchase Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Eligible to receive in cash |  |  |  |  |  |  |  |  |  | $ 6,000,000 |  |  |  |  |  |  |  |  |  |  |
Eligible to receive common stock, financing activities |  |  |  |  |  |  |  |  |  | $ 8,500,000 |  |  |  |  |  |  |  |  |  |  |
Eligible to receive shares of common stock | shares |  |  |  |  |  |  |  |  |  | 7,000,000 |  |  |  |  |  |  |  |  |  |  |
Gross proceeds from qualified financing |  |  | $ 20,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Additional common stock receive for qualified financing |  |  | $ 8,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock, trading period |  |  | 10 days |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock received | shares |  |  |  |  |  | 8,093,010 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash received |  |  |  |  |  | $ 500,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Reimbursable technology transfer expenses received |  |  |  |  |  | $ 300,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of performance obligations | Obligation |  |  |  |  |  | 1 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Upfront consideration to incremental discount for future payment |  |  |  |  |  | $ 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Remaining performance obligations |  |  |  |  |  | 0 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
License agreement transaction price |  |  |  |  |  | 1,800,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Value of common stock received |  |  |  |  |  | $ 1,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Doubtful revenue reversal of future cash payments |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 2,500,000
Revenue from contracts with customers |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 14,000,000 |  |  |  |  |  |
Contract assets |  | 0 |  |  |  |  |  |  |  |  |  | 0 |  |  | 0 |  |  | $ 0 |  |  |
Contract liabilities |  | 0 |  |  |  |  |  |  |  |  |  | 0 |  |  | 0 |  |  | 0 |  |  |
Capitalized contract costs |  | $ 0 |  |  |  |  |  |  |  |  |  | $ 0 |  |  | 0 |  |  | 0 |  |  |
Contract revenue |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 14,000,000 |  |  |  |  |  |
Consideration paid upon qualified financing event |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,500,000 |  |  |  |  |  |
Future cash payments of revenue recognized |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,500,000 |  |  |  |  |  |
Rezolute [Member] | License Agreement and Common Stock Purchase Agreement [Member] | Licenses and Related Materials, Product Data/Filing, Process and Know-how [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Recognized the entire transaction price as revenue upon completion of delivery |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 1,800,000 |  |  |
Rezolute [Member] | License Agreement and Common Stock Purchase Agreement [Member] | Forecast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Eligible to receive, in order to maintain license agreement | $ 15,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Future cash payments upon closing of the qualified financing |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | $ 8,500,000 |
Rezolute [Member] | Common Stock Purchase Agreement [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares of common stock held for future issuance | shares |  |  |  |  |  |  | 5,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock purchase agreement description |  |  |  |  |  |  |  |  |  |  |  |  |  |  | Under the common stock purchase agreement, Rezolute granted the Company the right and option to sell the greater of (i) 5,000,000 shares of common stock or (ii) one third of the aggregate shares held by the Company upon failure by Rezolute to list its shares of its common stock on the Nasdaq Stock Market or a similar national exchange on or prior to December 31, 2018. |  |  |  |  |  |
Percentage of aggregate shares held for future issuance |  |  |  |  |  |  | 33.33% |  |  |  |  |  |  |  |  |  |  |  |  |  |
Cash received |  |  |  |  |  |  |  | $ 5,500,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Reimbursable technology transfer expenses received |  |  |  |  |  |  |  | $ 300,000 |  |  |  |  |  |  |  |  |  |  |  |  |
Gross proceeds from preferred stock financing |  |  |  | $ 25,000,000 |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Rezolute [Member] | Common Stock Purchase Agreement [Member] | Interim Financing Closing and Initial Closing Financing Activities [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Common stock received | shares |  |  |  |  |  |  |  |  | 8,093,010 |  |  |  |  |  |  |  |  |  |  |  |
Cash received |  |  |  |  |  |  |  |  | $ 500,000 |  |  |  |  |  |  |  |  |  |  |  |
Rezolute [Member] | Common Stock Purchase Agreement [Member] | Forecast [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Maximum number of shares of common stock sold back | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 5,000,000 |  |  |  |
Rezolute [Member] | Common Stock Purchase Agreement [Member] | Forecast [Member] | Maximum [Member] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items] |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |
Number of shares of common stock sold back | shares |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  |  | 2,500,000 |  |  |  |

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- Definition Future cash payments received. + References No definition available. + Details
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- Definition Future cash payments upon closing of qualified financing. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
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- Definition Gross proceeds from qualified financing. + References No definition available. + Details
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Data Type: | xbrli:monetaryItemType
Balance Type: | debit
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- Definition License agreement transaction price. + References No definition available. + Details
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- Definition Number of future cash payments upon closing of qualified financing. + References No definition available. + Details
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- Definition Number of performance obligations. + References No definition available. + Details
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- Definition Percentage of aggregate shares held for future issuance. + References No definition available. + Details
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- Definition Percentage of decrease in future royalty obligations. + References No definition available. + Details
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- Definition Percentage of net proceeds of each future financing upon closing of the qualified financing. + References No definition available. + Details
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Namespace Prefix: | xoma_
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Balance Type: | na
Period Type: | duration

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- Definition Reimbursable technology transfer expenses received. + References No definition available. + Details
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- Definition Sale of stock in subsidiary maximum number of shares sold in transaction. + References No definition available. + Details
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- Definition Sale of stock in subsidiary number of shares sold in transaction. + References No definition available. + Details
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- Definition Shares received. + References No definition available. + Details
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Period Type: | duration

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- Definition Shares received value. + References No definition available. + Details
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Namespace Prefix: | xoma_
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Balance Type: | credit
Period Type: | duration

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- Definition Unrecognized future cash payments. + References No definition available. + Details
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Namespace Prefix: | xoma_
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Balance Type: | debit
Period Type: | duration

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- Definition Upfront consideration to incremental discount for future payment. + References No definition available. + Details
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Namespace Prefix: | xoma_
Data Type: | xbrli:monetaryItemType
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Period Type: | duration

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- Definition Upfront option payment. + References No definition available. + Details
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Namespace Prefix: | xoma_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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- Details
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