Collaboration Revenue - Schedule of Revenue Recognized (Detail) $ in Thousands	12 Months Ended
Dec. 31, 2018 USD ($)
ASU 2014-09 [Member]
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$ 15,774

Contract with customer liability revenue recognized excluding impact of revenue associated with milestones achieved.

No definition available.

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