Collaboration Revenue - Additional Information (Detail) - USD ($)	3 Months Ended			9 Months Ended		12 Months Ended
	Sep. 30, 2018	Sep. 30, 2017	Mar. 31, 2016	Sep. 30, 2018	Sep. 30, 2017	Dec. 31, 2017	Dec. 31, 2016	Jan. 01, 2018	Feb. 29, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront collaboration/license fee			$ 120,000,000	$ 120,000,000
Period of performance identifiable deliverables				10 years
Total reported liabilities	$ 139,607,000			$ 139,607,000		$ 128,823,000
Deferred revenue, net of current portion - related party	89,554,000			89,554,000		84,847,000
Deferred revenue - related party	17,901,000			17,901,000		12,079,000
Collaboration revenue - related party	8,684,000			16,721,000
Net Loss	(21,949,000)	$ (6,935,000)		(77,655,000)	$ (60,427,000)
Decrease in deferred revenue				(16,328,000)	(9,044,000)
Nestlé Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party	8,684,000	23,015,000		$ 16,721,000	29,044,000
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.
Transaction price allocated to remaining performance obligations	171,200,000			$ 171,200,000
Upfront cash payment	107,455,000			107,455,000					$ 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						20,000,000	$ 10,000,000
Deferred revenue	107,455,000			107,455,000
ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Deferred revenue, net of current portion - related party								$ 106,365,000
Deferred revenue - related party								17,418,000
Upfront cash payment	107,455,000	$ 99,770,000		107,455,000	$ 99,770,000	$ 123,783,000	$ 108,814,000
Difference between Revenue Guidance in Effect before and after Topic 606 [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Total reported liabilities	19,303,000			19,303,000
Deferred revenue, net of current portion - related party	(13,520,000)			(13,520,000)				21,518,000
Deferred revenue - related party	(5,783,000)			(5,783,000)				$ 5,339,000
Net Loss	(7,554,000)
Decrease in deferred revenue				(7,554,000)
Difference Between Proforma Value and Reported Value [Member] | ASU 2014-09 [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Collaboration revenue - related party	$ 5,600,000			$ 7,554,000