Collaboration Revenue - Additional Information (Detail)	1 Months Ended	3 Months Ended							12 Months Ended
	Mar. 31, 2019 USD ($) Installment	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Mar. 31, 2016 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)	Dec. 31, 2016 USD ($)	Nov. 30, 2018 USD ($)	Feb. 29, 2016 USD ($)	Jan. 31, 2016 USD ($)
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration/license fee								$ 120,000,000	$ 120,000,000
Collaboration revenue - related party									27,188,000	$ 26,917,000	$ 32,100,000
Collaboration revenue		$ 2,032,000	$ 2,106,000	$ 1,817,000	$ 260,000				6,215,000
Deferred revenue, current		4,834,000							4,834,000
Deferred revenue, non-current		4,834,000							4,834,000
Research Agreement [Member] | AstraZeneca Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment		9,668,000							9,668,000
Upfront collaboration milestone payments receivable	$ 20,000,000				$ 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									6,215,000
Deferred revenue, current		4,834,000							4,834,000
Deferred revenue, non-current		4,834,000							4,834,000
Nestlé Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment		110,071,000				$ 137,259,000			110,071,000	137,259,000				$ 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
Upfront collaboration milestone payments receivable						20,000,000				20,000,000
Proceeds on achievement of development milestone										40,000,000	20,000,000	$ 10,000,000
Transaction price allocated to remaining performance obligations		190,000,000				190,000,000			$ 190,000,000	190,000,000
Transaction price of milestone payment to be received							$ 20,000,000
Revenue information used to determine transaction price									The Company estimated the $20,000 of variable consideration by using the most likely amount method which best predicts the amount of consideration to which the Company will be entitled.
Contract modification catch-up adjustment						5,517,000
Collaboration revenue - related party									$ 27,188,000	26,917,000	$ 32,100,000
Deferred revenue		$ 110,071,000				$ 137,259,000			110,071,000	137,259,000
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
Collaboration revenue - related party									$ 6,830,000
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