Claim and Claim Adjustment Expense Reserves (Historical Claims Duration) (Details)	Dec. 31, 2024
Medical Professional Liability \| Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	3.60%
Year 2	16.30%
Year 3	21.80%
Year 4	16.60%
Year 5	12.70%
Year 6	8.90%
Year 7	5.70%
Year 8	3.20%
Year 9	1.10%
Year 10	1.60%
Other Professional Liability and Management Liability \| Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	6.50%
Year 2	19.60%
Year 3	19.20%
Year 4	14.70%
Year 5	10.50%
Year 6	6.90%
Year 7	5.60%
Year 8	6.10%
Year 9	2.50%
Year 10	0.60%
Surety \| Specialty
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	17.30%
Year 2	41.00%
Year 3	16.40%
Year 4	8.10%
Year 5	3.90%
Year 6	6.20%
Year 7	(2.30%)
Year 8	(0.30%)
Year 9	1.10%
Year 10	0.00%
Commercial Auto \| Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	23.70%
Year 2	22.30%
Year 3	18.40%
Year 4	14.60%
Year 5	10.50%
Year 6	5.10%
Year 7	2.40%
Year 8	0.90%
Year 9	0.50%
Year 10	1.10%
General Liability \| Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	3.40%
Year 2	12.50%
Year 3	15.70%
Year 4	16.80%
Year 5	13.80%
Year 6	9.20%
Year 7	7.10%
Year 8	5.70%
Year 9	3.20%
Year 10	1.20%
Workers' Compensation \| Commercial
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	16.80%
Year 2	23.40%
Year 3	14.20%
Year 4	8.80%
Year 5	5.90%
Year 6	3.80%
Year 7	2.30%
Year 8	1.20%
Year 9	1.00%
Year 10	0.60%
Excluding Hardy \| International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	16.20%
Year 2	21.00%
Year 3	9.60%
Year 4	6.50%
Year 5	6.10%
Year 6	6.50%
Year 7	7.50%
Year 8	2.70%
Year 9	3.90%
Year 10	2.30%
Hardy \| International
Short-duration Insurance Contracts, Historical Claims Duration [Line Items]
Year 1	15.40%
Year 2	28.90%
Year 3	15.30%
Year 4	8.60%
Year 5	5.20%
Year 6	6.20%
Year 7	1.80%
Year 8	3.30%
Year 9	(0.30%)
Year 10	(5.10%)

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.

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

- Definition

Percentage of average annual payout, after reinsurance, in the eighth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

Reference 1: http://www.xbrl.org/2003/role/exampleRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 55
-Paragraph 9F
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480046/944-40-55-9F

Reference 2: http://www.xbrl.org/2003/role/disclosureRef
-Topic 944
-SubTopic 40
-Name Accounting Standards Codification
-Section 50
-Paragraph 4G
-Publisher FASB
-URI https://asc.fasb.org/1943274/2147480081/944-40-50-4G

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearEight
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of average annual payout, after reinsurance, in the fifth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearFive
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of average annual payout, after reinsurance, in the fourth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearFour
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of average annual payout, after reinsurance, in the ninth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearNine
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of average annual payout, after reinsurance, in the first year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearOne
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of average annual payout, after reinsurance, in the seventh year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearSeven
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of average annual payout, after reinsurance, in the sixth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearSix
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of average annual payout, after reinsurance, in the tenth year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearTen
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of average annual payout, after reinsurance, in the third year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearThree
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Definition

Percentage of average annual payout, after reinsurance, in the second year after a claim is incurred, beginning with the earliest accident year disclosed for short-duration insurance contracts.

+ References

+ Details

Name:	us-gaap_ShortdurationInsuranceContractsHistoricalClaimsDurationYearTwo
Namespace Prefix:	us-gaap_
Data Type:	dtr-types:percentItemType
Balance Type:	na
Period Type:	instant

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cna_SpecialtySegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cna_CommercialSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

Name:	srt_ProductOrServiceAxis=us-gaap_GeneralLiabilityMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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

- Details

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

- Details

Name:	us-gaap_StatementBusinessSegmentsAxis=cna_InternationalSegmentMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

- Details

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