Related Party Transactions - Additional Information (Detail) - USD ($)		3 Months Ended		9 Months Ended
	Aug. 12, 2020	Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019	Dec. 31, 2019	Feb. 29, 2016
Related Party Transaction [Line Items]
Collaboration revenue - related party		$ 80,000	$ 4,840,000	$ 10,728,000	$ 21,909,000
Net proceeds from sale of common stock				244,020,000	60,527,000
Nestlé Health Science [Member]
Related Party Transaction [Line Items]
Collaboration revenue - related party		80,000	4,840,000	10,728,000	21,909,000
Deferred revenue		99,343,000		99,343,000		$ 110,071,000	$ 120,000,000
Payments under agreements with related party		0	$ 0	0	$ 0
Due from related party for the reimbursement of development costs		0		$ 0
Nestlé [Member] | Securities Purchase Agreement [Member] | Concurrent Placement [Member]
Related Party Transaction [Line Items]
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, shares	959,002
Common stock, share price	$ 20.855
Net proceeds from sale of common stock	$ 19,900,000
Flagship Pioneering [Member] | Sublease Agreement [Member]
Related Party Transaction [Line Items]
Sub lease commencement date				2019-07
Lessee, operating sublease, existence of option to extend				false
Lessee, operating sublease, option to extend, description				The term of the sublease agreement commenced in July 2019 and ends on the last day of the 24th calendar month following commencement, with no option to extend
Cash received from related party transaction		458,000		$ 1,357,000
Flagship Pioneering [Member] | Sublease Agreement [Member] | Other Income [Member]
Related Party Transaction [Line Items]
Sublease income		$ 458,000		$ 1,357,000