Commitments and Contingencies - Additional Information (Detail) - USD ($)			3 Months Ended		6 Months Ended		12 Months Ended
	May 28, 2019	Jan. 13, 2015	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020	Dec. 31, 2020	Dec. 31, 2017	Nov. 28, 2020	Oct. 22, 2019	Feb. 19, 2019	May 10, 2018	Aug. 24, 2004
Accrued Payments
Research and development expense			$ 13,570,000	$ 12,051,000	$ 26,906,000	$ 24,757,000
Cash balance			76,746,000		76,746,000		$ 115,069,000
Agreement commencement date								2015-05
Expected Cash Payment Payable									$ 1,500,000
Minimum Royalties Amount Payable									$ 100,000
Annual Licensing fee paid			0
Receivables			7,626,000		7,626,000		4,665,000
Restricted stock awards			$ 0		$ 1,000
MD Anderson Warrant [Member]
Accrued Payments
Number of Warrants			3,333,333		3,333,333
MD Anderson License and the Research and Development Agreement [Member]
Accrued Payments
Research and development service agreement aggregate quarterly payments					$ 15,000,000.0
Reimbursement of historical costs										$ 20,000,000.0
Aggregate potential benchmark payments										36,500,000
Accrued Payments										$ 3,000,000.0
Development costs incurred					0
CRADA Agreement [Member]
Accrued Payments
Obligations due under contract											$ 5,000,000.0
Quarterly payments under contract					1,300,000	1,300,000
CAR Products [Member]
Accrued Payments
Amount of royalties receivable												$ 50,000,000.0
The University of Texas MD Anderson Cancer Center and The Texas A & M University System
Accrued Payments
Milestone maximum payment													$ 4,500,000
Cash payment			$ 143,250		143,250
Restricted stock awards					917,000
Defined Benefit Plan, Expected Future Benefit Payment, Year Two			300,000		300,000
Defined Benefit Plan, Expected Future Benefit Payment, Year Three			300,000		300,000
MD Anderson License
Accrued Payments
Receivables			7,600,000		7,600,000		4,700,000
License Agreement with the National Cancer Institute [Member]
Accrued Payments
Minimum Royalties Amount Payable			300,000		300,000
Ziop License Agreement With The National Cancer Institute [Member]
Accrued Payments
Expected Cash Payment Payable	$ 1,500,000
Expected cash payment payable per installments	500,000
Minimum Royalties Amount Payable	$ 300,000
Description Of First Annual Royalty Payable	The first minimum annual royalty payment is payable on the date that is eighteen months following the date of the Patent License
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, which has not been met at June 30, 2021.
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
payments under the Patent License							1,500,000
Annual Licensing fee paid				0	0	0
Ziop License Agreement With The National Cancer Institute [Member] | Performance Based Payments Member [Member]
Accrued Payments
Aggregate Benchmark Payments Payable	$ 4,300,000
Ziop License Agreement With The National Cancer Institute [Member] | One Time Benchmark Payments [Member]
Accrued Payments
Potential Benchmark Payments Payable	12,000,000.0
Ziop License Agreement With The National Cancer Institute [Member] | Post Marketing Approval [Member] | Performance Based Payments Member [Member]
Accrued Payments
Aggregate Benchmark Payments Payable	3,000,000.0
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	$ 1,000,000,000.0
Patent And Technology License Agreement [Member] | The University of Texas MD Anderson Cancer Center and The Texas A & M University System
Accrued Payments
Accrued payments			0	0	0	0
Collaboration Agreement With Solasia Pharma KK [Member]
Accrued Payments
Accrued payments or receipts			0	0	0	0
License Agreement With PGEN Therapeutics [Member] | License [Member]
Accrued Payments
Cost of Goods and Services Sold			0	0	100,000	$ 0
Prepaid Expenses and Other Current Assets | MD Anderson License
Accrued Payments
Cash balance			1,800,000		1,800,000
Intrexon Corporation
Accrued Payments
Annual License Fees												100,000
Expected additional milestones payable												52,500,000
Expected Cash Payment Payable			100		100		$ 100
Intrexon Corporation | T-cell receptor
Accrued Payments
Maximum royalty amount												$ 100,000,000.0
Portion of income payable to related party												20.00%
Minimum | License Agreement with the National Cancer Institute [Member]
Accrued Payments
Royalty fee			100
Minimum | Ziop License Agreement With The National Cancer Institute [Member]
Accrued Payments
Royalty fee				$ 300,000
Maximum [Member] | MD Anderson License and the Research and Development Agreement [Member]
Accrued Payments
Research and development expense		$ 20,000,000.0
Maximum [Member] | The University of Texas MD Anderson Cancer Center and The Texas A & M University System
Accrued Payments
Defined Benefit Plan, Expected Future Benefit Payment, Year One			$ 600,000		$ 600,000