Segment Reporting - Schedule of reportable segment net income (loss) (Detail) - USD ($) $ in Thousands		3 Months Ended
		Mar. 31, 2025	Mar. 31, 2024
Segment Reporting Information [Line Items]
Operating Expenses		$ 27,236	$ 34,438
Gain on sale of VOWST Business		(52,181)	0
Net income (loss) from continuing operations		32,682	(32,903)
Net (loss) from discontinuing operations, net of tax		0	(7,230)
Net income (loss)		32,682	(40,133)
Operating Segments [Member]
Segment Reporting Information [Line Items]
R&D personnel-related (including stock-based compensation)		6,118	7,845
G&A personnel-related (including stock-based compensation)		4,173	6,641
Professional fees		3,180	2,628
Facility-related and other		4,535	5,675
Gain on sale of VOWST Business		(52,181)	0
Other segment expense (income)	[1]	(4,207)	(1,450)
Net income (loss) from continuing operations		32,682	(32,903)
Net (loss) from discontinuing operations, net of tax	[2]	0	(7,230)
Net income (loss)		32,682	(40,133)
Live Biotherapeutics Platform [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating Expenses		5,123	9,164
SER-155 [Member] | Operating Segments [Member]
Segment Reporting Information [Line Items]
Operating Expenses		$ 577	$ 2,400

[1] Other segment (expense) income includes research and development expenses on early stage programs, manufacturing services expenses, interest income and other (expense) income, net.

[2] See Note 3, Discontinued Operations and TSA, for further details.