Commitments and Contingencies - Additional Information (Details) - USD ($)			3 Months Ended			9 Months Ended		12 Months Ended
	May 28, 2019	Jan. 13, 2015	Sep. 30, 2021	Mar. 31, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 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			$ 14,521,000		$ 13,968,000	$ 41,427,000	$ 38,725,000
Cash balance			91,725,000			91,725,000		$ 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			1,510,000			$ 1,510,000		4,665,000
Restricted stock awards			$ 0
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
Development costs paid						0
CRADA Agreement [Member]
Accrued Payments
Obligations due under contract												$ 5,000,000.0
Quarterly payments under contract			$ 0		600,000	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
MD Anderson License
Accrued Payments
Receivables			1,100,000			1,100,000		4,700,000
Accrued expenses			2,100,000			2,100,000		1,400,000
License Agreement with the National Cancer Institute [Member]
Accrued Payments
Minimum Royalties Amount Payable			300,000			300,000
payments under the Patent License				$ 100,000
License Agreement with the National Cancer Institute [Member] | One Time Benchmark Payments [Member]
Accrued Payments
Accrued payments or receipts			100,000			100,000
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 September 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			80,000			80,000
Accrued payments					0		0
Collaboration Agreement With Solasia Pharma KK [Member]
Accrued Payments
Accrued payments or receipts			400,000		0	400,000	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			0			0
Intrexon Corporation
Accrued Payments
Annual License Fees													100,000
Expected additional milestones payable													52,500,000
Expected Cash Payment Payable								$ 100,000
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			$ 300,000		$ 0		$ 0
Minimum | Ziop License Agreement With The National Cancer Institute [Member]
Accrued Payments
Royalty fee						$ 500,000
Maximum [Member] | MD Anderson License and the Research and Development Agreement [Member]
Accrued Payments
Research and development expense		$ 20,000,000.0