Significant agreements - Revenue recognition due to changes in contract assets and liabilities (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2021	Mar. 31, 2020
Significant Agreements
Revenue recognized based on proportional performance	$ 1,652	$ 1,129
Revenue recognized based on changes in transaction price	156
Total	$ (1,808)	$ (1,129)

Amount of revenue recognized based on proportional performance that was previously included in balance of obligation to transfer good or service to customer for which consideration from customer has been received or is due.

No definition available.

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