Defined Contribution Plan - Narrative (Details) - USD ($) $ in Millions	3 Months Ended	9 Months Ended
Sep. 30, 2020	Sep. 30, 2019	Sep. 30, 2020	Sep. 30, 2019
Retirement Benefits [Abstract]
Vesting period			4 years
Employer contribution made	$ 0.1	$ 0.1	$ 0.1	$ 0.1

Period which an employee's right to exercise a plan is no longer contingent on satisfaction of either a service condition, market condition or a performance condition, in 'PnYnMnDTnHnMnS' format, for example, 'P1Y5M13D' represents the reported fact of one year, five months, and thirteen days.

No definition available.

Name:	selb_DefinedContributionPlanEmployersMatchingContributionVestingPeriod
Namespace Prefix:	selb_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration