Leases - Summary of Future Undiscounted Cash Inflows Under the Sublease (Detail) $ in Thousands	Sep. 30, 2019 USD ($)
Leases [Abstract]
2019 (remaining 3 months)	$ 307
2020	1,248
2021	633
Future undiscounted cash inflows	$ 2,188

Lessee operating sublease payments to be received reminder of fiscal year.

No definition available.

Name:	mcrb_LesseeOperatingSubleasePaymentsToBeReceivedReminderOfFiscalYear
Namespace Prefix:	mcrb_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant