CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (Unaudited) (Parenthetical) $ in Thousands	3 Months Ended
Mar. 31, 2020 USD ($)
Statement of Stockholders' Equity [Abstract]
Issuance of common stock, issuance costs	$ 98

Stock Issued During Period, New Issues, Issuance Costs

No definition available.

Name:	sesn_StockIssuedDuringPeriodNewIssuesIssuanceCosts
Namespace Prefix:	sesn_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration