Collaboration Revenue - Additional Information (Detail)				1 Months Ended		3 Months Ended	12 Months Ended
	Jul. 21, 2021 USD ($)	Jul. 01, 2021 USD ($)	Jan. 31, 2016 USD ($)	Dec. 31, 2020 USD ($)	Mar. 31, 2019 USD ($) Installment	Dec. 31, 2020 USD ($)	Dec. 31, 2021 USD ($)	Dec. 31, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 30, 2018 USD ($)	Feb. 29, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration profit (loss)							$ 1,732,000
Upfront collaboration/license fee			$ 120,000,000
Collaboration revenue - related party							$ 143,857,000	$ 11,897,000	$ 27,188,000
Collaboration revenue								17,161,000	6,215,000
2021 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration product, percentage of commercial profit		50.00%
Collaborative arrangement, sharing of pre-launch costs, percentage		50.00%
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		$ 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	$ 20,000,000
Research and development estimated reimbursement costs							13,900,000
Reimbursement of research and development costs incurred								3,376,000
Collaboration revenue						15,145,000	0
Nestle Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment				$ 108,174,000		$ 108,174,000	97,728,000	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								10,000,000		$ 40,000,000	$ 20,000,000	$ 10,000,000
Transaction price allocated to remaining performance obligations							200,000,000
Collaboration revenue - related party							10,446,000	$ 11,897,000	27,188,000
Deferred revenue							103,817,000		$ 108,174,000
Nestle Health Science [Member] | 2021 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration product, percentage of commercial profit		50.00%
Upfront payment received	$ 175,000,000
Maximum amount to be received on achievement of regulatory and sales milestones		$ 360,000,000
Maximum amount to be received on achievement of sales milestones		225,000,000
Collaboration profit (loss)							1,732,000
Maximum regulatory payments to be received		$ 135,000,000
Transaction price allocated to remaining performance obligations	$ 139,500,000
Termination notice period	60 days
Nestle Health Science [Member] | 2021 License Agreement [Member] | License [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party							131,343,000
Nestle Health Science [Member] | 2021 License Agreement [Member] | Service [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party							2,068,000
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
Topic 808 [Member] | Nestle Health Science [Member] | 2021 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Transaction price allocated under collaborative arrangement	$ 35,500,000
Topic 808 [Member] | Total Liabilities [Member] | Nestle Health Science [Member] | 2021 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Transaction price allocated under collaborative arrangement	$ 35,500,000
Topic 808 [Member] | Other Long-term Liabilities [Member] | Nestle Health Science [Member] | 2021 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Transaction price allocated under collaborative arrangement							10,585,000
Topic 808 [Member] | Accrued Expenses and Other Current Liabilities [Member] | Nestle Health Science [Member] | 2021 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Transaction price allocated under collaborative arrangement							21,098,000
Research and development expenses [Member] | Topic 808 [Member] | Nestle Health Science [Member] | 2021 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cost associated with pre-launch activities							2,168,000
General and administrative expenses [Member] | Topic 808 [Member] | Nestle Health Science [Member] | 2021 License Agreement [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Cost associated with pre-launch activities							$ 3,383,000