Commitments and Contingencies - Additional Information (Detail) - USD ($) $ in Thousands				1 Months Ended	3 Months Ended		9 Months Ended
	May 28, 2019	Aug. 17, 2015	Jan. 13, 2015	Jul. 31, 2015	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Jan. 01, 2021	Nov. 28, 2020	Sep. 28, 2020	Dec. 31, 2019	Oct. 22, 2019	Feb. 19, 2019	Oct. 05, 2018	Aug. 24, 2004
Accrued Payments
Research and development expense					$ 13,968	$ 8,641	$ 38,725	$ 28,115
Cash balance					135,471		135,471					$ 79,741
Agreement commencement date		2015-05
MD Anderson License and the Research and Development Agreement Member [Member]
Accrued Payments
Research and development service agreement aggregate quarterly payments							15,000
Reimbursement of historical costs													$ 20,000
Accrued Payments													3,000
Number of Warrants									3,333,333
CRADA Agreement [Member]
Accrued Payments
Obligations due under contract														$ 5,000
Quarterly payments under contract					600	600	1,300	1,300
Aggregate potential benchmark payments													$ 36,500
CAR Products [Member]
Accrued Payments
Amount of royalties receivable															$ 50,000
ZIOP License Agreement With The National Cancer Institute [Member]
Accrued Payments
Expected cash payment payable per installments	$ 500
The University of Texas MD Anderson Cancer Center and The Texas A & M University System
Accrued Payments
Milestone maximum payment																$ 4,500
Obligations due under contract					400		400				$ 400
ZIOP License Agreement With The National Cancer Institute [Member]
Accrued Payments
Expected Cash Payment Payable	1,500
Minimum Royalties Amount Payable	$ 300
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.
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					0	0	0	0
Payments under patent license							1,500
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] | Scenario, Forecast [Member]
Accrued Payments
Expected Cash Payment Payable										$ 1,500
Minimum Royalties Amount Payable										$ 100
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] | License [Member]
Accrued Payments
Licensing fee					0		600
Payments under patent license							600
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
Prepaid Expenses and Other Current Assets | MD Anderson License
Accrued Payments
Cash balance					11,200		11,200
Prepaid Expenses and Other Current Assets | ZIOP License Agreement With The National Cancer Institute [Member]
Accrued Payments
Prepaid Royalties					300		300
Intrexon Corporation
Accrued Payments
Licensing fee				$ 115,000
Research and development expense						$ 300		$ 1,700
Milestone payment receivable					5,000		5,000
Upfront payment received				$ 57,500
Percentage of upfront fee Payable				50.00%
Annual Licensing fee															100
Expected additional milestones payable															52,500
Expected Cash Payment Payable					$ 0		0					$ 100
Annual Licensing fee paid							$ 100
Intrexon Corporation | T-cell receptor
Accrued Payments
Maximum royalty amount															$ 100,000
Portion of income payable to related party															20.00%
Maximum | MD Anderson License and the Research and Development Agreement Member [Member]
Accrued Payments
Research and development expense			$ 20,000