Commitments and Contingencies - Narrative (Details) - USD ($) $ in Thousands	3 Months Ended
Mar. 31, 2025	Dec. 31, 2024
Commitments and Contingencies
Financing liability	$ 640	$ 925
Loss contingency, estimate of possible loss	1,500	1,200
Penn \| Penn License Agreement
Commitments and Contingencies
Annual payments to be made	25
Additional milestone payments for first CAR-M product	1,700
Penn \| Penn License Agreement \| Maximum
Commitments and Contingencies
Milestone payments	10,900
Additional milestone payments	30,000
Accounts Payable
Commitments and Contingencies
Loss contingency, estimate of possible loss	1,000
Accrued Expenses
Commitments and Contingencies
Loss contingency, estimate of possible loss	500
Laboratory Equipment
Commitments and Contingencies
Financing liability	$ 1,700	$ 1,400

Additional Milestone Payments for the First CAR-M Product

No definition available.

Name:	carm_AdditionalMilestonePaymentsForTheFirstCARMProduct
Namespace Prefix:	carm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Additional Milestone Payments Required to be Paid

+ References

No definition available.

+ Details

Name:	carm_AdditionalMilestonePaymentsRequiredToBePaid
Namespace Prefix:	carm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Amount Payable upon Achievement of Certain Clinical, Regulatory and Commercial Milestone Events

+ References

No definition available.

+ Details

Name:	carm_AmountPayableUponAchievementOfCertainClinicalRegulatoryAndCommercialMilestoneEvents
Namespace Prefix:	carm_
Data Type:	xbrli:monetaryItemType
Balance Type:	credit
Period Type:	instant

- Definition

Commitments Annual Payments Required To Be Paid

+ References

No definition available.

+ Details

Name:	carm_CommitmentsAnnualPaymentsRequiredToBePaid
Namespace Prefix:	carm_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Present value of lessee's discounted obligation for lease payments from finance lease.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 6
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147478964/842-20-50-6

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 842
-SubTopic 20
-Name Accounting Standards Codification
-Section 45
-Paragraph 1
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147479041/842-20-45-1

+ Details

Name:	us-gaap_FinanceLeaseLiability
Namespace Prefix:	us-gaap_
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

Reference 1: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-4

Reference 2: http://www.xbrl.org/2009/role/commonPracticeRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (a)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-9

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 720
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483359/720-20-50-1

Reference 4: http://www.xbrl.org/2003/role/exampleRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 27
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482395/460-10-55-27

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-9

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-4

Reference 7: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-3

Reference 8: http://www.xbrl.org/2003/role/disclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-1

+ Details

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

- Definition

Reflects the estimated amount of loss from the specified contingency as of the balance sheet date.

+ References

Reference 1: http://www.xbrl.org/2003/role/disclosureRef
-Topic 720
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 1
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483359/720-20-50-1

Reference 2: http://www.xbrl.org/2003/role/exampleRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 55
-Paragraph 27
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482395/460-10-55-27

Reference 3: http://www.xbrl.org/2003/role/disclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 9
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-9

Reference 4: http://www.xbrl.org/2003/role/disclosureRef
-Topic 450
-SubTopic 20
-Name Accounting Standards Codification
-Section 50
-Paragraph 4
-Subparagraph (b)
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147483076/450-20-50-4

Reference 5: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 3
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-3

Reference 6: http://www.xbrl.org/2003/role/disclosureRef
-Topic 460
-SubTopic 10
-Name Accounting Standards Codification
-Section 50
-Paragraph 2
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147482425/460-10-50-2

+ Details

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

- Details

Name:	us-gaap_RelatedPartyTransactionAxis=carm_TrusteesOfUniversityOfPennsylvaniaMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_TypeOfArrangementAxis=carm_PennLicenseAgreementMember
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_BalanceSheetLocationAxis=us-gaap_AccountsPayableMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_BalanceSheetLocationAxis=us-gaap_AccruedLiabilitiesMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	us-gaap_PropertyPlantAndEquipmentByTypeAxis=us-gaap_EquipmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: