Collaboration Revenue - Schedule of Revenue Recognized (Detail) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2019	Mar. 31, 2018
Revenue recognized in the period from:
Amounts included in the contract liability at the beginning of the period	$ 6,615	$ 3,766

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