Collaboration Revenue - Additional Information (Detail)	1 Months Ended		3 Months Ended			6 Months Ended	9 Months Ended		12 Months Ended
	Mar. 31, 2019 USD ($) Installment	Dec. 31, 2018 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2018 USD ($)	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]
Collaboration revenue - related party			$ 4,840,000		$ 8,684,000		$ 21,909,000	$ 16,721,000
Collaboration revenue			2,106,000				4,183,000
Deferred revenue, current			1,790,000				1,790,000
Deferred revenue, non-current			2,410,000				2,410,000
Research Agreement [Member] | AstraZeneca Inc. [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment			4,200,000				4,200,000
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			2,106,000				4,183,000
Deferred revenue, current			1,790,000				1,790,000
Deferred revenue, non-current			2,410,000				2,410,000
Nestlé Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment			115,350,000				115,350,000						$ 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												$ 40,000,000
Proceeds on achievement of development milestone		$ 40,000,000							$ 40,000,000	$ 20,000,000	$ 10,000,000
Transaction price allocated to remaining performance obligations			190,000,000				190,000,000
Collaboration revenue - related party			4,840,000		$ 8,684,000		21,909,000	$ 16,721,000
Deferred revenue		$ 137,259,000	$ 115,350,000				$ 115,350,000		$ 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		$ 6,830,000