Collaboration Revenue - Schedule of Revenue Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$ 6,081	$ 11,837	$ 21,909

Contract with customer liability revenue recognized with related party.

No definition available.

Name:	mcrb_ContractWithCustomerLiabilityRevenueRecognizedWithRelatedParty
Namespace Prefix:	mcrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration