Summary of significant accounting policies - Government grants (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2019	Jun. 30, 2018	Jun. 30, 2019	Jun. 30, 2018
Summary of significant accounting policies
Reduction of research and development	$ 0.1	$ 0.0	$ 0.2	$ 0.0

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