Nature of the Business and Basis of Presentation - Additional Information (Detail) $ / shares in Units, $ in Thousands			3 Months Ended						9 Months Ended
	Aug. 12, 2020 USD ($) $ / shares shares	Aug. 10, 2020 Patient	Sep. 30, 2020 USD ($) shares	Jun. 30, 2020 USD ($)	Mar. 31, 2020 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($) shares	Mar. 31, 2019 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Dec. 31, 2019 USD ($)
Nature Of Business And Basis Of Presentation [Line Items]
Entity incorporated month and year									2010-10
Entity incorporation, state or country code									DE
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs									$ 244,020	$ 60,527
Accumulated deficit			$ 530,524						530,524		$ 459,649
Net loss			(30,280)	$ (20,714)	$ (19,881)	$ (16,409)	$ (10,759)	$ (24,333)	(70,875)	(51,501)
Cash used in operations									(75,683)	$ (62,473)
Cash, cash equivalents and investments			$ 320,312						$ 320,312
Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, shares | shares			12,075,000				28,818,578
Underwritten Public Offering [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs	$ 243,748
Underwritten Public Offering [Member] | Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, shares | shares	10,500,000
Common stock, share price | $ / shares	$ 21.50
Concurrent Placement [Member] | Securities Purchase Agreement [Member] | Nestlé [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Issuance of common stock from public offering, net of commissions, underwriting discounts and offering costs, shares | shares	959,002
Common stock, share price | $ / shares	$ 20.855
Proceeds from public offering of common stock, net of commissions, underwriting discounts and offering costs	$ 19,900
Maximum [Member] | Underwritten Public Offering [Member] | Common Stock [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Option to purchase additional shares of common stock | shares	1,575,000
SER-109 [Member] | Minimum [Member]
Nature Of Business And Basis Of Presentation [Line Items]
Number of patients | Patient		300