Selected Quarterly Financial Data (Unaudited) - Summary of Quarterly Financial Information (Parenthetical) (Detail) - USD ($) $ in Thousands	1 Months Ended		3 Months Ended		12 Months Ended
	Feb. 29, 2016	Jan. 31, 2016	Sep. 30, 2017	Sep. 30, 2016	Dec. 31, 2017	Dec. 31, 2016
Quarterly Financial Information [Line Items]
Performance period for revenue recognized		10 years
Nestlé Health Science [Member]
Quarterly Financial Information [Line Items]
Upfront cash payment	$ 120,000	$ 120,000			$ 120,000
Proceeds on achievement of development milestone			$ 20,000	$ 10,000	20,000	$ 10,000
Collaboration revenue recognized for milestone - related party					$ 20,000	$ 10,000
Performance period for revenue recognized		10 years