Represents the threshold period of notice required for termination of agreement.

No definition available.

Name:	bcyc_CollaborativeArrangementNoticeRequiredForTerminationOfAgreement
Namespace Prefix:	bcyc_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount to be paid upon exercise of an option by the customer.

+ References

No definition available.

+ Details

Name:	bcyc_CustomerOptionPayment
Namespace Prefix:	bcyc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Amount of milestone payment under revenue from contract with customer.

+ References

No definition available.

+ Details

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

- Definition

Line items represent financial concepts included in a table. These concepts are used to disclose reportable information associated with domain members defined in one or many axes to the table.

+ References

No definition available.

+ Details

Name:	us-gaap_CollaborativeArrangementsAndNoncollaborativeArrangementTransactionsLineItems
Namespace Prefix:	us-gaap_
Data Type:	xbrli:stringItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of revenue recognized 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.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (b)
-URI http://asc.fasb.org/extlink&oid=123351226&loc=SL49130549-203045

+ Details

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Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Definition

Amount, excluding tax collected from customer, of revenue from satisfaction of performance obligation by transferring promised good or service to customer. Tax collected from customer is tax assessed by governmental authority that is both imposed on and concurrent with specific revenue-producing transaction, including, but not limited to, sales, use, value added and excise.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 280
-SubTopic 10
-Section 50
-Paragraph 30
-Subparagraph (a)
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-Section S99
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-URI http://asc.fasb.org/extlink&oid=6472922&loc=d3e499488-122856

+ Details

Name:	us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	duration

- Details

Name:	us-gaap_TypeOfArrangementAxis=bcyc_OxurionCollaborationAgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	bcyc_AchievementOfMilestoneAxis=bcyc_DevelopmentMilestoneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	bcyc_AchievementOfMilestoneAxis=bcyc_RegulatoryMilestoneMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	bcyc_AchievementOfMilestoneAxis=bcyc_GrantingFirstRegulatoryApprovalOrFirstIndicationMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Significant agreements - Oxurion Collaboration Agreement (Details) $ in Thousands, € in Millions	3 Months Ended		9 Months Ended			12 Months Ended
	Sep. 30, 2021 USD ($)	Sep. 30, 2020 USD ($)	Sep. 30, 2021 USD ($)	Sep. 30, 2021 EUR (€)	Sep. 30, 2020 USD ($)	Dec. 31, 2020 USD ($)
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Revenue from collaborative arrangement \| $	$ 4,333	$ 3,842	$ 7,926		$ 6,542
Oxurion
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Threshold period of notice required for termination of agreement			90 days	90 days
Revenue recognized \| $		2,400			2,400
Revenue from collaborative arrangement \| $	$ 0	$ 2,362	$ 0		$ 2,362	$ 0
Oxurion \| Development milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone payments				€ 8.3
Revenue under contracts with customer from milestone payment				3.8
Oxurion \| Regulatory milestone
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone payments				16.5
Oxurion \| Granting first regulatory approval or first indication
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Potential milestone payments				€ 5.0