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License, Clinical Trial and Sponsored Research Agreements (Sponsored Research and Clinical Trial Agreements) (Details) - USD ($)	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	$ 14,083,000	$ 7,987,000	$ 36,603,000	$ 27,131,000
Sponsored Research and Clinical Trial Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	1,756,000	734,000	4,858,000	4,900,000
City of Hope National Medical Center ("COH") [Member] | Sponsored Research and Clinical Trial Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	0	0	0	500,000
Mayo Clinic [Member] | Sponsored Research and Clinical Trial Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	231,000	0	464,000	0
CD123 (MB-102) [Member] | City of Hope National Medical Center ("COH") [Member] | Sponsored Research and Clinical Trial Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	24,000	48,000	250,000	344,000
IL13Ra2 (MB-101) [Member] | City of Hope National Medical Center ("COH") [Member] | Sponsored Research and Clinical Trial Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	199,000	96,000	992,000	422,000
CS1 (MB-104) [Member] | City of Hope National Medical Center ("COH") [Member] | Sponsored Research and Clinical Trial Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	138,000	65,000	510,000	835,000
HER2 (MB 103) [Member] | City of Hope National Medical Center ("COH") [Member] | Sponsored Research and Clinical Trial Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	319,000	0	473,000	0
PSCA (MB-105) [Member] | City of Hope National Medical Center ("COH") [Member] | Sponsored Research and Clinical Trial Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	23,000	0	69,000	0
Research and Development (Income) Expenses	23,000
CD20 (MB-106) [Member] | Fred Hutchinson Cancer Research Center [Member] | Sponsored Research and Clinical Trial Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	492,000	418,000	1,490,000	1,134,000
ST. Jude - XSCID [Member] | St. Jude Children's Research Hospital [Member] | Sponsored Research and Clinical Trial Agreements [Member]
Collaborative Arrangements and Non-collaborative Arrangement Transactions [Line Items]
Research and development expense	$ 330,000	$ 107,000	$ 610,000	$ 1,665,000

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Research and Development Expenses - Sponsored Research and Clinical Trial Agreements

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The costs incurred in a planned search or critical investigation aimed at discovery of new knowledge with the hope that such knowledge will be useful in developing a new product or service, a new process or technique, or in bringing about a significant improvement to an existing product or process; or to translate research findings or other knowledge into a plan or design for a new product or process or for a significant improvement to an existing product or process whether intended for sale or the entity's use, during the reporting period charged to research and development projects, excluding in-process research and development acquired in a business combination consummated during the period. Excludes software research and development, which has a separate concept.

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Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 730
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-Section 50
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