Collaboration Revenue - Additional Information (Detail)	1 Months Ended		3 Months Ended		12 Months Ended
	Dec. 31, 2020 USD ($)	Mar. 31, 2019 USD ($) Installment	Mar. 31, 2021 USD ($)	Mar. 31, 2020 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	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			$ 4,648,000	$ 5,462,000
Collaboration revenue				1,988,000
Research Agreement [Member] | AstraZeneca Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable		$ 20,000,000
Transaction price allocated to remaining performance obligations	$ 23,377,000				$ 23,377,000
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
Termination date	Apr. 02, 2021
Termination notice period	120 days
Research and development fixed consideration	$ 20,000,000		$ 20,000,000		20,000,000
Research and development estimated reimbursement costs			13,900,000
Reimbursement of research and development costs incurred					3,376,000
Collaboration revenue			0	1,988,000	15,145,000
Nestle Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	108,174,000		103,526,000		108,174,000					$ 120,000,000
Maximum development milestone payments to be received			285,000,000
Maximum regulatory payments to be received			375,000,000
Maximum amount to be received on achievement of certain commercial milestones			1,125,000,000
Proceeds on achievement of development milestone						$ 40,000,000	$ 20,000,000	$ 10,000,000
Transaction price allocated to remaining performance obligations			200,000,000
Transaction price of milestone payment to be received					10,000,000
Collaboration revenue - related party			4,648,000	$ 5,462,000
Deferred revenue	$ 108,174,000		$ 103,526,000		$ 108,174,000
Nestle Health Science [Member] | Phase 2 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable											$ 20,000,000
Nestle Health Science [Member] | Phase 3 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration milestone payments receivable											$ 20,000,000
Nestle 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