Leases - Summary of Future Undiscounted Cash Inflows Under the Sublease (Detail) $ in Thousands	Dec. 31, 2019 USD ($)
Leases [Abstract]
2020	$ 1,248
2021	633
Future undiscounted cash inflows	$ 1,881

Lessee operating sublease payments to be received next twelve months.

No definition available.

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