Claim and Claim Adjustment Expense Reserves (Commercial - Detail of Develpment Recorded) (Details) - Commercial - USD ($) $ in Millions	3 Months Ended
Mar. 31, 2018	Mar. 31, 2017
Liability for Claims and Claims Adjustment Expense [Line Items]
Commercial Auto	$ (1)	$ (26)
General Liability	(8)	(18)
Workers' Compensation	(6)	0
Property and Other	6	1
Total pretax (favorable) unfavorable development	$ (9)	$ (43)

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

No definition available.

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_CommercialMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: