v3.25.3

Claim and Claim Adjustment Expense Reserves (Commercial - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development) (Details) - Commercial - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2025	Sep. 30, 2024	Sep. 30, 2025	Sep. 30, 2024
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial	$ 0	$ 25	$ 50	$ 46
General Liability	0	28	62	47
Workers' Compensation	(1)	(57)	(66)	(106)
Property and Other	0	1	0	2
Total pretax (favorable) unfavorable development	$ (1)	$ (3)	$ 46	$ (11)

- Definition

Net prior year claim and allocated claim adjustment expense reserve development, Commercial Auto

+ References

No definition available.

+ Details

Name:	cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentCommercialAuto
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Net prior year claim and allocated claim adjustment expense reserve development, General Liability

+ References

No definition available.

+ Details

Name:	cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentGeneralLiability
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Net prior year claim and allocated claim adjustment expense reserve development, Property and Other

+ References

No definition available.

+ Details

Name:	cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentPropertyAndOther
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Net prior year claim and allocated claim adjustment expense reserve development, Workers' Compensation

+ References

No definition available.

+ Details

Name:	cna_NetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentWorkersCompensation
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

- Definition

Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group

+ References

No definition available.

+ Details

Name:	cna_PretaxFavorableUnfavorableNetPriorYearClaimAndAllocatedClaimAdjustmentExpenseReserveDevelopmentExcludingLifeGroup
Namespace Prefix:	cna_
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

No definition available.

+ Details

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

- Details

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

CNA FINANCIAL CORP 10-Q