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

The amount of upfront cash payment received from collaborative arrangement.

+ References

No definition available.

+ Details

Name:	bcyc_CollaborativeArrangementUpfrontCashPaymentReceived
Namespace Prefix:	bcyc_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
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 obligation to transfer good or service to customer for which consideration has been received or is receivable.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 1
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+ Details

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

- 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

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-Publisher FASB
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+ 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

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+ Details

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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:
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Period Type:

Significant agreements - Oxurion Collaboration Agreement (Details) $ in Thousands, € in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 EUR (€)	Sep. 30, 2020 USD ($)	Sep. 30, 2019 USD ($)	Sep. 30, 2020 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)
Significant agreements.
Revenue from collaborative arrangement	$ 3,842	$ 614		$ 6,542	$ 8,520
Deferred revenue						$ 33,275	$ 5,657	$ 14,635
Revenue recognized	1,267	173		$ 3,496	5,596
Oxurion
Significant agreements.
Threshold period of notice required for termination of agreement			90 days	90 days
Revenue from collaborative arrangement	2,362	$ 0		$ 2,362	$ 0
Deferred revenue						$ 0
Revenue recognized	$ 2,400			$ 2,400
Oxurion \| Development milestone
Significant agreements.
Upfront cash payment \| €			€ 1.0
Potential milestone payments \| €			8.3
Revenue under contracts with customer from milestone payment \| €			3.8
Oxurion \| Regulatory milestone
Significant agreements.
Potential milestone payments \| €			16.5
Oxurion \| Granting first regulatory approval or first indication
Significant agreements.
Potential milestone payments \| €			€ 5.0