Summary of Significant Accounting Policies - Research and development incentives and receivable (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Research and development incentives and receivable
Maximum percentage of surrenderable losses	14.50%
Reduction of research and development	$ 9.1	$ 6.7	$ 5.9

Maximum percentage of surrenderable trading losses that arise from qualifying research and development expenses incurred by the Company’s subsidiaries in the United Kingdom for a tax credit.

No definition available.

Name:	bcyc_MaximumPercentageOfSurrenderableLosses
Namespace Prefix:	bcyc_
Data Type:	num:percentItemType
Balance Type:	na
Period Type:	duration