CONDENSED CONSOLIDATED STATEMENTS OF CHANGES IN STOCKHOLDERS' EQUITY (DEFICIT) (Parenthetical) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2020	Mar. 31, 2020	Jun. 30, 2019	Jun. 30, 2020
Statement of Stockholders' Equity [Abstract]
Issuance of common stock, issuance costs	$ 0.1	$ 0.1	$ 2.2	$ 0.2

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