Aggregate collaboration transaction amount.

No definition available.

Name:	ctmx_AggregateCollaborationTransactionAmount
Namespace Prefix:	ctmx_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Future contingent milestone payment receivable.

+ References

No definition available.

+ Details

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

- Definition

Estimated research service period.

+ References

No definition available.

+ Details

Name:	ctmx_EstimatedResearchServicePeriod
Namespace Prefix:	ctmx_
Data Type:	xbrli:durationItemType
Balance Type:	na
Period Type:	duration

- Definition

Estimated research service termination month and year.

+ References

No definition available.

+ Details

Name:	ctmx_EstimatedResearchServiceTerminationMonthAndYear
Namespace Prefix:	ctmx_
Data Type:	xbrli:gYearMonthItemType
Balance Type:	na
Period Type:	duration

- Definition

Research and development service fees.

+ References

No definition available.

+ Details

Name:	ctmx_ResearchAndDevelopmentServiceFees
Namespace Prefix:	ctmx_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 808
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Subparagraph (d)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479402/808-10-50-1

+ Details

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

- Definition

Amount, after allowance for credit loss, of right to consideration in exchange for good or service transferred to customer when right is conditioned on something other than passage of time.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-11

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-3

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-1

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-8

+ Details

Name:	us-gaap_ContractWithCustomerAssetNet
Namespace Prefix:	us-gaap_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

- 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
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 11
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-11

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-1

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 8
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479806/606-10-50-8

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 606
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479837/606-10-45-2

+ 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
-Topic 230
-SubTopic 10
-Name Accounting Standards Codification
-Section 45
-Paragraph 25
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482740/230-10-45-25

+ Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_TypeOfArrangementAxis=ctmx_CollaborationAndLicenseAgreementMember
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_ContingentConsiderationByTypeAxis=ctmx_AdditionalContingentPaymentsMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

Collaboration and License Agreements - Regeneron Pharmaceuticals, Inc - Additional Information (Details) - USD ($) $ in Thousands	Nov. 16, 2022	Dec. 31, 2025	Dec. 31, 2024	Dec. 31, 2023
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue		$ 28,467	$ 94,063	$ 212,315
Regeneron Pharmaceuticals, Inc
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Deferred revenue		10,500
Collaborative Arrangement \| Regeneron Pharmaceuticals, Inc
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront payment received	$ 30,000
Collaboration and License Agreement \| Regeneron Pharmaceuticals, Inc
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Upfront payment received	30,000
Total transaction price	39,200
Research and development service fees	$ 9,200
Estimated research service period	4 years
Estimated research service termination month & year	2026-11
Deferred revenue		10,500	15,600
Amount due from customer		$ 800	$ 1,000
Collaboration and License Agreement \| Regeneron Pharmaceuticals, Inc \| Maximum
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contingent milestone payments receivable	$ 800,000
Collaboration and License Agreement \| Regeneron Pharmaceuticals, Inc \| Maximum \| Additional Contingent Payments
Collaborative Arrangement and Arrangement Other than Collaborative [Line Items]
Contingent milestone payments receivable	$ 1,200,000