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
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130531-203044

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 8
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130549-203045

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 45
-Paragraph 2
-URI http://asc.fasb.org/extlink&oid=118944033&loc=SL49130532-203044

+ Details

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

- 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 606
-SubTopic 10
-Section 50
-Paragraph 4
-Subparagraph (a)
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130543-203045

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 606
-SubTopic 10
-Section 50
-Paragraph 5
-URI http://asc.fasb.org/extlink&oid=118956577&loc=SL49130545-203045

+ 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:

Significant agreements - Oxurion Collaboration Agreement (Details) $ in Thousands, € in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 EUR (€)	Jun. 30, 2019 USD ($)	Jun. 30, 2018 USD ($)	Jun. 30, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant agreements
Revenue from collaborative arrangement	$ 1,522	$ 1,661		$ 7,906	$ 4,469
Deferred revenue						$ 10,370	$ 14,635	$ 14,467
Oxurion
Significant agreements
Threshold period of notice required for termination of agreement			90 days	90 days
Revenue from collaborative arrangement	$ 0	177		$ 0	1,559
Deferred revenue						$ 0	$ 0
Oxurion \| Development milestone
Significant agreements
Upfront cash payment \| €			€ 1.0
Potential milestone payments \| €			8.3
Revenue under contracts with customer from milestone payment \| €			1.8
Revenue from collaborative arrangement		$ 1,200			$ 1,200
Oxurion \| Regulatory milestone
Significant agreements
Potential milestone payments \| €			€ 16.5