Collaboration Revenue - Additional Information (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		9 Months Ended		12 Months Ended
	Feb. 29, 2016	Jan. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2016
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Period of performance identifiable deliverables		10 years
Collaboration revenue - related party			$ 23,015	$ 13,015	$ 29,044	$ 18,730
Nestlé Health Science [Member]
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront cash payment	$ 120,000				120,000
Development milestone payments		$ 285,000
Regulatory payments		375,000
Sales based milestones		$ 1,125,000
Reimbursement of development services					580
Proceeds on achievement of development milestone			20,000				$ 10,000
Collaboration revenue associated with substantive milestones - related party			20,000				$ 10,000
Collaboration revenue - related party			23,015	$ 13,015	29,044	$ 18,730
Deferred revenue			$ 99,770		$ 99,770