Leases - Schedule Of Subleasing (Details) $ in Thousands	Mar. 31, 2023 USD ($)
Leases [Abstract]
Remainder of 2023	$ 974
2024	1,333
2025	1,379
2026	1,067
Total sublease income payments	$ 4,753

Lessee operating leases future minimum payments due future minimum sublease rentals.

No definition available.

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

Lessee operating leases future minimum payments due future minimum sublease rentals due in next twelve months.

No definition available.

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

Lessee operating leases future minimum payments due future minimum sublease rentals due in year three.

No definition available.

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

Lessee operating leases future minimum payments due future minimum sublease rentals due in year two.

No definition available.

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

Lessee operating leases future minimum payments due future minimum sublease rentals remainder of fiscal year.

No definition available.

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

No definition available.