Summary of significant accounting policies - Government grants (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2023	Dec. 31, 2022	Dec. 31, 2021
Summary of significant accounting policies
Reduction of research and development related to government grant arrangements	$ 1.0	$ 1.5	$ 3.0
Government grant funding remaining for future cost reimbursement	$ 0.1

Amount of reduction of research and development expense related to government grant arrangements.

No definition available.

Name:	bcyc_ReductionOfResearchAndDevelopmentExpenseRelatedToGovernmentGrantArrangements
Namespace Prefix:	bcyc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration