Significant agreements - Revenue recognition due to changes in contract assets and liabilities (Details) - USD ($) $ in Thousands	3 Months Ended	9 Months Ended
Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Significant agreements
Revenue recognized based on proportional performance	$ 3,206	$ 1,267	$ 6,263	$ 3,496
Revenue recognized based on expiration of material rights	1,127		1,507
Revenue recognized based on changes in transaction price			156
Total	$ 4,333	$ 1,267	$ 7,926	$ 3,496

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