Commitments and Contingencies - Additional Information (Detail) - USD ($)			1 Months Ended	12 Months Ended
	May 28, 2019	Jan. 13, 2015	Jul. 31, 2015	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017	Nov. 28, 2020	Oct. 22, 2019	Feb. 19, 2019	Nov. 11, 2018	Dec. 31, 2014	Aug. 24, 2004
Accrued Payments
Research and development expense				$ 52,696,000	$ 38,331,000	$ 34,134,000
Cash balance				115,069,000	79,741,000
Milestone payment receivable				$ 57,500,000
Agreement commencement date							2015-05
Expected Cash Payment Payable								$ 1,500,000
Minimum Royalties Amount Payable								$ 100,000
Number of Warrants											18,939,394
MD Anderson Warrant [Member]
Accrued Payments
Number of Warrants				3,333,333
MD Anderson License and the Research and Development Agreement Member [Member]
Accrued Payments
Research and development service agreement aggregate quarterly payments				$ 15,000,000.0
Reimbursement of historical costs									$ 20,000,000.0
Accrued Payments									3,000,000.0
Aggregate potential benchmark payments									$ 36,500,000
CRADA Agreement [Member]
Accrued Payments
Obligations due under contract										$ 5,000,000.0
Quarterly payments under contract				2,500,000	2,500,000
The University of Texas MD Anderson Cancer Center and The Texas A & M University System
Accrued Payments
Milestone maximum payment													$ 4,500,000
Payments for Royalties				400,000
The University of Texas MD Anderson Cancer Center and The Texas A & M University System | Upon enrollment of the first patient in a multi-center pivotal clinical trial
Accrued Payments
Shares vested												50,222
ARES Trading License
Accrued Payments
Research and development expense						$ 100,000
License Agreement with the National Cancer Institute [Member]
Accrued Payments
payments under the Patent License				0
Ziop License Agreement With The National Cancer Institute [Member]
Accrued Payments
Minimum Royalties Amount Payable	$ 300,000
Description Of First Annual Royalty Payable	The first minimum annual royalty payment was paid during the year ending December 31, 2020
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
payments under the Patent License				500,000	1,000,000.0
Accrued Payments				0	$ 500,000
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] | License [Member]
Accrued Payments
payments under the Patent License				600,000
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	$ 1,000,000,000.0
Prepaid Expenses and Other Current Assets | MD Anderson License
Accrued Payments
Cash balance				8,100,000
Prepaid Expenses and Other Current Assets | Ziop License Agreement With The National Cancer Institute [Member]
Accrued Payments
Prepaid Royalties				300,000
Intrexon Corporation
Accrued Payments
Licensing fee			$ 115,000,000.0
Milestone payment receivable				$ 5.0
Upfront payment received			$ 57,500,000
Percentage of upfront fee Payable			50.00%
Maximum [Member] | MD Anderson License and the Research and Development Agreement Member [Member]
Accrued Payments
Research and development expense		$ 20,000,000.0