Research and Development Agreements - Additional Information (Detail) - USD ($) $ / shares in Units, $ in Thousands	1 Months Ended			3 Months Ended			6 Months Ended		107 Months Ended
	Nov. 30, 2019	Jul. 31, 2019	Sep. 30, 2018	Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020	Jun. 30, 2019	Nov. 30, 2019	Dec. 31, 2019
Research And Development Collaboration Agreements [Line Items]
Common stock, shares issued				45,778,876			45,778,876			45,589,157
Common stock, par value				$ 0.001			$ 0.001			$ 0.001
Revenue from grant				$ 6,909		$ 5,447	$ 11,050	$ 12,455
Grant
Research And Development Collaboration Agreements [Line Items]
Revenue from grant				869		$ 236	3,210	$ 831
CPRIT Agreement | Cancer Prevention and Research Institute of Texas
Research And Development Collaboration Agreements [Line Items]
Product development grant awarded			$ 15,200
CPRIT Agreement | Cancer Prevention and Research Institute of Texas | ETB MT-3724
Research And Development Collaboration Agreements [Line Items]
Aggregate proceeds received from award granted									$ 10,600
Revenue from grant							14,000
Grants Receivable				5,900			5,900
Grant Agreements | Grants Revenue Receivable
Research And Development Collaboration Agreements [Line Items]
Reimbursement amounts submitted in excess of amounts received are recorded as receivables				5,900			5,900			$ 7,100
Takeda Development Agreement
Research And Development Collaboration Agreements [Line Items]
Total transaction price							29,800
Upfront payment					$ 30,000
Development milestone payment that is achievable							10,000
Expected co-share payments payable				10,200			10,200
Deferred revenue				4,200			4,200			6,100
Takeda Development Agreement | Clinical Development and Regulatory Milestones
Research And Development Collaboration Agreements [Line Items]
Milestone payments receivable if option is exercised		$ 307,500
Takeda Development Agreement | Sales Milestone
Research And Development Collaboration Agreements [Line Items]
Milestone payments receivable if option is exercised		$ 325,000
Takeda Multi Target Agreement | Takeda
Research And Development Collaboration Agreements [Line Items]
Deferred revenue				2,900			2,900			3,100
Cumulative payments received							5,000
Aggregate milestone payments upon exercise of option to license ETBS				30,000			30,000
Additional preclinical, clinical development and commercialization milestone payment				397,000			397,000
Takeda Multi Target Agreement | Takeda | Maximum
Research And Development Collaboration Agreements [Line Items]
Contractual contingency fees							10,000
Commercial milestone payment							150,000
Vertex Collaboration Agreement
Research And Development Collaboration Agreements [Line Items]
Milestone payments receivable if option is exercised	$ 180,000
Upfront payment	38,000
Deferred revenue				$ 23,000			$ 23,000			$ 27,500
Upfront payment, cash	23,000
Upfront payment, equity method investments	$ 15,000								$ 15,000
Common stock, shares issued	1,666,666								1,666,666
Common stock, par value	$ 0.001								$ 0.001
Vertex Collaboration Agreement | Common Stock
Research And Development Collaboration Agreements [Line Items]
Purchase price per share	$ 9.00								$ 9.00
Fair value of allocated consideration	$ 4,500
Vertex Collaboration Agreement | ETBs
Research And Development Collaboration Agreements [Line Items]
Aggregate milestone payments upon exercise of option to license ETBS	22,000								$ 22,000
Vertex Collaboration Agreement | Sales Milestone
Research And Development Collaboration Agreements [Line Items]
Milestone payments receivable if option is exercised	$ 70,000