Future contingent milestone payment receivable.

No definition available.

Name:	ctmx_ContingentMilestonePaymentsReceivable
Namespace Prefix:	ctmx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Contingent payments receivable profit split on sales.

+ References

No definition available.

+ Details

Name:	ctmx_ContingentPaymentsReceivableProfitSplitOnSales
Namespace Prefix:	ctmx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	duration

- Definition

Contingent payments receivable upon achieving development, regulatory and commercial milestones.

+ References

No definition available.

+ Details

Name:	ctmx_ContingentPaymentsReceivableUponAchievingDevelopmentRegulatoryAndCommercialMilestones
Namespace Prefix:	ctmx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- Definition

Milestone payment received.

+ References

No definition available.

+ Details

Name:	ctmx_MilestonePaymentReceived
Namespace Prefix:	ctmx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Milestone payments received from revenue recognized during period.

+ References

No definition available.

+ Details

Name:	ctmx_MilestonePaymentsReceivedFromRevenueRecognizedDuringPeriod
Namespace Prefix:	ctmx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Number of collaboration agreements.

+ References

No definition available.

+ Details

Name:	ctmx_NumberOfCollaborationAgreements
Namespace Prefix:	ctmx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of targets.

+ References

No definition available.

+ Details

Name:	ctmx_NumberOfTargets
Namespace Prefix:	ctmx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Number of targets selected.

+ References

No definition available.

+ Details

Name:	ctmx_NumberOfTargetsSelected
Namespace Prefix:	ctmx_
Data Type:	xbrli:integerItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of net Income or net losses related to development and commercialization costs.

+ References

No definition available.

+ Details

Name:	ctmx_PercentageOfNetIncomeOrNetLossesRelatedToDevelopmentAndCommercializationCosts
Namespace Prefix:	ctmx_
Data Type:	dtr-types:percentItemType
Balance Type:	na
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 https://asc.fasb.org/extlink&oid=126919976&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 https://asc.fasb.org/extlink&oid=126920106&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 https://asc.fasb.org/extlink&oid=126919976&loc=SL49130532-203044

+ Details

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

- Definition

Cash received from collaborators during the current period.

+ References

Reference 1: http://fasb.org/us-gaap/role/ref/legacyRef
-Publisher FASB
-Name Accounting Standards Codification
-Topic 230
-SubTopic 10
-Section 45
-Paragraph 25
-Subparagraph (a)
-URI https://asc.fasb.org/extlink&oid=126954810&loc=d3e3536-108585

+ Details

Name:	us-gaap_ProceedsFromCollaborators
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Indicates type of revenue from product and service. Includes, but is not limited to, revenue from contract with customer and other sources.

+ References

No definition available.

+ Details

Name:	us-gaap_TypeOfRevenueExtensibleList
Namespace Prefix:	us-gaap_
Data Type:	enum2:enumerationSetItemType
Balance Type:	na
Period Type:	duration

- Details

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

- Details

Name:	srt_CounterpartyNameAxis=ctmx_AbbVieIrelandUnlimitedCompanyMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=ctmx_CD71AgreementMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_StatementGeographicalAxis=country_US
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_RangeAxis=srt_MaximumMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

Name:	srt_ProductOrServiceAxis=ctmx_SecondTargetMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_ContingentConsiderationByTypeAxis=ctmx_DevelopmentRegulatoryAndCommercialMilestonePaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Research and Collaboration Agreements - AbbVie Ireland Unlimited Company - Additional Information (Details) $ in Thousands	1 Months Ended		3 Months Ended				6 Months Ended
	Jun. 30, 2019 USD ($)	Apr. 30, 2016 USD ($) Agreement Target	Jun. 30, 2022 USD ($)	Mar. 31, 2022	Jun. 30, 2021	Mar. 31, 2021	Jun. 30, 2022 USD ($)	Jun. 30, 2021	Dec. 31, 2021 USD ($)	Mar. 31, 2017 Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Type of Revenue [Extensible List]			us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember	us-gaap:LicenseAndServiceMember
Deferred revenue			$ 161,650				$ 161,650		$ 194,922
Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of collaboration agreements \| Agreement		2
AbbVie Ireland Unlimited Company
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payment received							100,000
AbbVie Ireland Unlimited Company \| Collaborative Arrangement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Percentage of net profits or net losses related to development and commercialization costs		35.00%
AbbVie Ireland Unlimited Company \| CD71 Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received		$ 20,000
Deferred revenue			9,500				9,500		16,100
AbbVie Ireland Unlimited Company \| CD71 Agreement \| U.S
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Profit split on U.S. sales		35.00%
AbbVie Ireland Unlimited Company \| CD71 Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent payments receivable upon achieving development, regulatory and commercial milestones		$ 470,000
AbbVie Ireland Unlimited Company \| Discovery Agreement
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Upfront payment received		$ 10,000
Number of targets selected \| Target										1
Deferred revenue			$ 4,000				$ 4,000		$ 5,200
AbbVie Ireland Unlimited Company \| Discovery Agreement \| Second Target
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Milestone payments received	$ 10,000
AbbVie Ireland Unlimited Company \| Discovery Agreement \| Maximum
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Number of targets \| Target		2
AbbVie Ireland Unlimited Company \| Discovery Agreement \| Maximum \| Development, Regulatory and Commercial Milestone Payments
Collaborative Arrangements And Noncollaborative Arrangement Transactions [Line Items]
Contingent milestone payments receivable		$ 265,000