The amount of non refundable payment received for services during the BV Bicycle Research Term.

No definition available.

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

- Definition

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

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of option fee payable in order to obtain screening services.

+ References

No definition available.

+ Details

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

- Definition

Represents the amount of option fee payable in order to obtain worldwide development and exploitation rights.

+ References

No definition available.

+ Details

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

- Definition

Represents the renewal period of Research.

+ References

No definition available.

+ Details

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

- Definition

Represents the term of research program.

+ References

No definition available.

+ Details

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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 of revenue recognized based on expiration of material rights 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

No definition available.

+ Details

Name:	bcyc_ContractWithCustomerLiabilityRevenueRecognizedExpirationOfMaterialRights
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

Represents the number of FTE required to be funded by counterparty.

+ References

No definition available.

+ Details

Name:	bcyc_NumberOfFte
Namespace Prefix:	bcyc_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	duration

- Definition

Amount of allocated determined transaction price under revenue recognition arrangement.

+ References

No definition available.

+ Details

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

- 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

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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

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Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	bcyc_PerformanceObligationAxis=bcyc_HemophiliaResearchLicenseAndRelatedServicesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	bcyc_PerformanceObligationAxis=bcyc_SickleCellLicenseOptionMaterialRightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	bcyc_PerformanceObligationAxis=bcyc_HemophiliaLicenseOptionMaterialRightMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=bcyc_SanofiCollaborationAgreementsMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	bcyc_PerformanceObligationAxis=bcyc_ProgramThreeMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	srt_RangeAxis=srt_MinimumMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

Name:	bcyc_AchievementOfMilestoneAxis=bcyc_DevelopmentMilestoneMember
Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Namespace Prefix:
Data Type:	na
Balance Type:
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- Details

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Data Type:	na
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- Details

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Significant agreements - Sanofi Collaboration Agreement (Details) $ in Thousands	1 Months Ended	3 Months Ended								12 Months Ended
	Aug. 31, 2017 USD ($) employee	Dec. 31, 2019 USD ($)	Sep. 30, 2019 USD ($)	Jun. 30, 2019 USD ($)	Mar. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Sep. 30, 2018 USD ($)	Jun. 30, 2018 USD ($)	Mar. 31, 2018 USD ($)	Dec. 31, 2019 USD ($)	Dec. 31, 2018 USD ($)	Dec. 31, 2017 USD ($)
Significant agreements.
Transaction price	$ 14,200
Revenue recognized by non exercising of option										$ 9,984
Revenue from collaborative arrangement		$ 5,281	$ 614	$ 1,522	$ 6,384	$ 1,057	$ 1,610	$ 1,661	$ 2,808	13,801	$ 7,136	$ 2,060
Deferred revenue		5,657				14,635				5,657	14,635	14,467
Sickle Cell Research License and Related Services
Significant agreements.
Transaction price	1,405
Hemophilia Research License and Related Services
Significant agreements.
Transaction price	2,811
Sickle Cell License Option Material Right
Significant agreements.
Transaction price	5,286
Hemophilia License Option Material Right
Significant agreements.
Transaction price	4,698
Sanofi
Significant agreements.
Option fee payment to obtain the screening services	$ 5,000
Term of research program	3 years
Renewal period of Research	1 year
Upfront cash payment	$ 10,000
Non refundable payment for services during the BV Bicycle Research Term	$ 4,200
Threshold period of notice required for termination of agreement	30 days
Transaction price	$ 14,200	14,900								14,900
Revenue from collaborative arrangement										10,724	4,007	355
Deferred revenue		$ 0				$ 9,908				0	$ 9,908	$ 14,467
Sanofi \| Sickle Cell Research License and Related Services
Significant agreements.
Non refundable payment for services during the BV Bicycle Research Term	1,400
Revenue recognized by non exercising of option										5,300
Sanofi \| Hemophilia Research License and Related Services
Significant agreements.
Non refundable payment for services during the BV Bicycle Research Term	2,800
Sanofi \| Hemophilia License Option Material Right
Significant agreements.
Revenue recognized by non exercising of option										$ 4,700
Sanofi \| Program 3
Significant agreements.
Non refundable payment for services during the BV Bicycle Research Term	$ 1,400
Sanofi \| Minimum
Significant agreements.
Number of FTE \| employee	2
Sanofi \| Development milestone
Significant agreements.
Option fee payable in order to obtain worldwide development and exploitation rights	$ 5,000
Sanofi \| Development milestone \| Sickle Cell License Option Material Right
Significant agreements.
Potential milestone payments	47,500
Sanofi \| Development milestone \| Hemophilia License Option Material Right
Significant agreements.
Potential milestone payments	67,000
Sanofi \| Regulatory milestone
Significant agreements.
Potential milestone payments	104,000
Sanofi \| Commercial milestone
Significant agreements.
Potential milestone payments	55,000
Sanofi \| Commercialization license per candidate
Significant agreements.
Potential milestone payments	$ 5,000