Commitments and Contingencies - Additional Information (Details) - USD ($) $ / shares in Units, $ in Thousands					3 Months Ended		12 Months Ended
	May 28, 2019	Aug. 17, 2015	Jan. 13, 2015	Jan. 13, 2015	Mar. 31, 2022	Mar. 31, 2021	Dec. 31, 2019	Oct. 22, 2019	Feb. 19, 2019	Oct. 05, 2018	Aug. 24, 2004
Accrued Payments
Research and development expense					$ 5,580	$ 13,336
MD Anderson Warrant [Member]
Accrued Payments
Number of Warrants								3,333,333
Warrant exercise per share								$ 0.001
Warrant Expiry date								Dec. 31, 2026
MD Anderson Agreement and Worldwide License [Member]
Accrued Payments
Agreement commencement date			2015-05
MD Anderson License and the Research and Development Agreement [Member]
Accrued Payments
Reimbursement of historical costs								$ 20,000
Incurred expenses					400	0
Aggregate potential benchmark payments								36,500
Payments due prior to the first marketing approval								$ 3,000
CRADA Agreement [Member]
Accrued Payments
Incurred expenses					0	600
Obligations due under contract									$ 5,000
CAR Products [Member]
Accrued Payments
Maximum Royalty Amount					100,000
The University of Texas MD Anderson Cancer Center and The Texas A & M University System
Accrued Payments
Milestone maximum payment											$ 4,500
Payment under license agreement					0	0
License Agreement with the National Cancer Institute [Member] | One Time Benchmark Payments [Member]
Accrued Payments
First benchmark payment	$ 100
Ziop License Agreement With The National Cancer Institute [Member]
Accrued Payments
Maximum royalties amount payable	300
Royalties amount payable thereafter	100
Expected Cash Payment Payable	$ 1,500
Description Of First Benchmark Payable	The first benchmark payment of $0.1 million will be due upon the initiation of the Company's first sponsored Phase 1 clinical trial of a licensed product or licensed process in the field of use licensed under the Patent License.
Description Of option To terminate Agreement	The NCI may terminate or modify the Patent License in the event of a material breach, including if the Company does not meet certain milestones by certain dates, or upon certain insolvency events that remain uncured following the date that is 90 days following written notice of such breach or insolvency event.
Agreement termination, notice period	60 days
Annual Licensing fee paid							$ 1,000
Ziop License Agreement With The National Cancer Institute [Member] | Performance Based Payments Member [Member]
Accrued Payments
Aggregate Benchmark Payments Payable	$ 4,300
Ziop License Agreement With The National Cancer Institute [Member] | One Time Benchmark Payments [Member]
Accrued Payments
Potential Benchmark Payments Payable	12,000
Ziop License Agreement With The National Cancer Institute [Member] | Post Marketing Approval [Member] | Performance Based Payments Member [Member]
Accrued Payments
Aggregate Benchmark Payments Payable	3,000
Ziop License Agreement With The National Cancer Institute [Member] | licensed products [Member] | One Time Benchmark Payments [Member]
Accrued Payments
Maximum Sales Revenue On Which Benchmark Payments Payable	1,000,000
Minimum sales revenue on which benchmark payments payable	$ 250,000
Collaboration Agreement With Solasia Pharma KK [Member]
Accrued Payments
Collaboration revenue					0	0
Intrexon Corporation
Accrued Payments
Annual License Fees										$ 100
Expected additional milestones payable										52,500
Intrexon Corporation | T-cell receptor
Accrued Payments
Maximum royalty amount										$ 100,000
Portion of income payable to related party										20.00%
Minimum | MD Anderson License and the Research and Development Agreement [Member]
Accrued Payments
Research and development expense		$ 15,000
Minimum | License Agreement with the National Cancer Institute [Member]
Accrued Payments
License Expense					$ 300	$ 300
Maximum [Member] | MD Anderson License and the Research and Development Agreement [Member]
Accrued Payments
Research and development expense				$ 20,000