Related Party Transactions - Additional Information (Detail) - USD ($)		3 Months Ended				12 Months Ended
		Dec. 31, 2016	Sep. 30, 2016	Jun. 30, 2016	Mar. 31, 2016	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Related Party Transaction [Line Items]
Collaboration revenue - related party	[1]	$ 3,037,000	$ 13,015,000	$ 3,004,000	$ 2,710,000	$ 21,766,000
Flagship Pioneering [Member] | Services Agreement [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party						17,000	$ 502,000	$ 454,000
Due to related party		0				0	$ 0
Nestlé Health Science [Member]
Related Party Transaction [Line Items]
Payments under agreements with related party						0
Due to related party		306,000				306,000
Collaboration revenue - related party						21,766,000
Deferred revenue		108,814,000				108,814,000
Due from related party for the reimbursement of development costs		$ 306,000				$ 306,000

[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 million, 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 million from NHS related to achievement of a milestone within this agreement, which was recognized in full as related party collaboration revenue during 2016.