Related Party Transactions - Additional Information (Detail) - USD ($)			3 Months Ended		12 Months Ended
	Oct. 05, 2018	Jun. 29, 2016	Mar. 31, 2019	Mar. 31, 2018	Dec. 31, 2018	Dec. 31, 2015
Related Party Transaction [Line Items]
Change in fair value of derivative liability				$ 28,000
Amounts expensed for services incurred			$ 13,621,000	16,342,000
Research and development expense			9,476,000	10,183,000	$ 1,000,000
Aggregate Amount Potentially Reimburseable Amount	$ 1,000,000
Related Party Transaction Expenses From Transactions With Related Party Advisory Fees					$ 2,000,000
MD Anderson License and the Research and Development Agreement Member [Member]
Related Party Transaction [Line Items]
Research and development service agreement aggregate quarterly payments			1,400,000
Cash resources on hand			26,400,000
MD Anderson License and the Research and Development Agreement Member [Member] | Other Current Assets [Member]
Related Party Transaction [Line Items]
Cash resources on hand			19,800,000
MD Anderson License and the Research and Development Agreement Member [Member] | Other Noncurrent Assets [Member]
Related Party Transaction [Line Items]
Cash resources on hand			6,600,000
Gorilla IL-12 Products
Related Party Transaction [Line Items]
Percentage of development cost shared by other party	80.00%
Percentage of operating profit shared by other party	80.00%
Series 1 Preferred Stock
Related Party Transaction [Line Items]
Temporary equity, fair value				5,100,000
Change in fair value of derivative liability			(165,000)	28,000
Related Party Transaction Expenses From Transactions With Related Party Advisory Fees	$ 5,400,000
License Agreement | M.D. Anderson Cancer Center
Related Party Transaction [Line Items]
Issuance of common stock in licensing agreement (in shares)						11,722,163
Research and development expense						$ 67,300,000
Cooperative Research and Development Agreement | M.D. Anderson Cancer Center
Related Party Transaction [Line Items]
Research and development service agreement aggregate quarterly payments				2,700,000
Research and development service agreement aggregate payments				41,900,000
Intrexon Corporation/Precigen
Related Party Transaction [Line Items]
Amounts expensed for services incurred			1,200,000	2,800,000
Amount due to related party, current			$ 900,000	$ 2,800,000
Aggregate Amount Potentially Reimburseable Amount	1,000,000
Annual License Fees	100,000
Additional Milestone Payment Payable For Licensed Program	52,500,000
Maximum Royalty payable	$ 100,000,000
Percentage Of Sublicensing Income	20.00%
Maximum Royalty Receivables	$ 50,000,000
Preferred Stock Contract Liability Derivative Liability	163,300,000
Increase in Accumulated Deficit	49,500,000
Related Party Transaction, Amounts of Transaction	$ 212,800,000
Intrexon Corporation/Precigen | Gorilla IL-12 Products
Related Party Transaction [Line Items]
Percentage of development cost shared by other party	20.00%
Percentage of operating profit shared by other party	20.00%
Intrexon Corporation/Precigen | Series 1 Preferred Stock
Related Party Transaction [Line Items]
Issuance of common stock in licensing agreement (in shares)		100,000		3,624
Consideration Transferred in License Agreement	$ 207,300,000
Preferred Stock Redemption Discount	5,400,000
Third Party Vendor [Member]
Related Party Transaction [Line Items]
Related Party Transaction Expenses From Transactions With Related Party Advisory Fees	$ 7,400,000