Collaboration Revenue - Additional Information (Detail)	1 Months Ended	3 Months Ended		9 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) Installment	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Dec. 31, 2019 USD ($)	Nov. 30, 2018 USD ($)	Feb. 29, 2016 USD ($)	Jan. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration/license fee				$ 120,000,000
Collaboration revenue - related party		$ 80,000	$ 4,840,000	10,728,000	$ 21,909,000
Collaboration revenue			2,106,000	2,016,000	4,183,000
Deferred revenue, current		6,784,000		6,784,000					$ 4,834,000
Deferred revenue, non-current		1,695,000		1,695,000					4,834,000
Research Agreement [Member] | AstraZeneca Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment		8,479,000		8,479,000
Upfront collaboration milestone payments receivable	$ 20,000,000
Transaction price allocated to remaining performance obligations		33,900,000		$ 33,900,000
Minimum exclusivity period	3 years
Number of installments | Installment	3
Option exercise period from exclusivity period				90 days
Terms of the definitive license agreement good faith negotiation period				6 months
Other terms of definitive license agreement period				1 year
Research and development fixed consideration		20,000,000		$ 20,000,000
Research and development estimated reimbursement costs				13,900,000
Collaboration revenue		0	2,106,000	2,016,000	4,183,000
Deferred revenue, current		6,784,000		6,784,000
Deferred revenue, non-current		1,695,000		1,695,000
Nestlé Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment		99,343,000		99,343,000					110,071,000		$ 120,000,000
Maximum development milestone payments to be received		285,000,000		285,000,000
Maximum regulatory payments to be received		375,000,000		375,000,000
Maximum amount to be received on achievement of certain commercial milestones		1,125,000,000		$ 1,125,000,000
Proceeds on achievement of development milestone						$ 40,000,000	$ 20,000,000	$ 10,000,000
Transaction price of milestone payment to be received		10,000,000
Revenue information used to determine transaction price				The Company estimated the $10,000 of variable consideration by using the most likely amount method which best predicts the amount of consideration to which the Company will be entitled.
Cumulative catch-up revenue		4,565,000		$ 4,565,000
Transaction price allocated to remaining performance obligations		200,000,000		200,000,000
Collaboration revenue - related party		80,000	$ 4,840,000	10,728,000	$ 21,909,000
Deferred revenue		$ 99,343,000		$ 99,343,000					$ 110,071,000
Nestlé Health Science [Member] | Phase 2 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable												$ 20,000,000
Nestlé Health Science [Member] | Phase 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable												$ 20,000,000
Nestlé Health Science [Member] | Phase 2b [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable										$ 40,000,000
Proceeds on achievement of development milestone						$ 40,000,000