Related Party Transactions - Additional Information (Detail) - USD ($)		3 Months Ended								12 Months Ended
		Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Related Party Transaction [Line Items]
Collaboration revenue - related party	[1]	$ 3,056,000	$ 23,015,000	$ 3,014,000	$ 3,015,000	$ 3,037,000	$ 13,015,000	$ 3,004,000	$ 2,710,000	$ 32,100,000	$ 21,766,000
Flagship Pioneering [Member] | Services Agreement [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party										0	17,000	$ 502,000
Due to related party		0				$ 0				0	0
Nestlé Health Science [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party										0
Collaboration revenue - related party										32,100,000	$ 21,766,000
Deferred revenue		96,926,000								96,926,000
Due from related party for the reimbursement of development costs		$ 510,000								$ 510,000

[1] In January 2016, the Company entered into a Collaboration and License Agreement with NHS for the development and commercialization of certain of its product candidates. In exchange for the license, NHS paid the Company an upfront cash payment of $120,000, which the Company received in February 2016 and is being recognized as revenue over the ten year performance period. During the quarter ended September 30, 2016, the Company received $10,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016. During the quarter ended September 30, 2017, the Company received $20,000 from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2017.