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Collaboration Agreements - AskBio (Narrative) (Details) - USD ($) $ in Thousands | 3 Months Ended
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Mar. 31, 2021 | Mar. 31, 2020
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |
Operating expenses | $ 18,208 | $ 18,822
AskBio License |  |
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items] |  |
Operating expenses |  | $ 1,200
Cost share percentage | 50.00% |

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- Definition Collaborative Arrangement, Cost Share Percentage + References No definition available. + Details
Name: | selb_CollaborativeArrangementCostSharePercentage
Namespace Prefix: | selb_
Data Type: | num:percentItemType
Balance Type: | na
Period Type: | duration

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- Definition Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table. + References No definition available. + Details
Name: | us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix: | us-gaap_
Data Type: | xbrli:stringItemType
Balance Type: | na
Period Type: | duration

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- Definition Generally recurring costs associated with normal operations except for the portion of these expenses which can be clearly related to production and included in cost of sales or services. Includes selling, general and administrative expense. + References No definition available. + Details
Name: | us-gaap_OperatingExpenses
Namespace Prefix: | us-gaap_
Data Type: | xbrli:monetaryItemType
Balance Type: | debit
Period Type: | duration

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- Details
Name: | us-gaap_TypeOfArrangementAxis=selb_AskBioLicenseMember
Namespace Prefix: |
Data Type: | na
Balance Type: |
Period Type: |