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Reinsurance (Voluntary Reinsurance) (Details) $ in Millions	Dec. 31, 2024 USD ($)
Effects of Reinsurance [Line Items]
Total voluntary reinsurance outstanding balance	$ 5,491
A- to A++
Effects of Reinsurance [Line Items]
Total voluntary reinsurance outstanding balance	4,585
B- to B++
Effects of Reinsurance [Line Items]
Total voluntary reinsurance outstanding balance	892
Insolvent
Effects of Reinsurance [Line Items]
Total voluntary reinsurance outstanding balance	$ 14

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- Definition

Voluntary Reinsurance Recoverable, Gross

+ References

No definition available.

+ Details

Name:	cna_VoluntaryReinsuranceRecoverableGross
Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	instant

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

X

- Details

Name:	cna_CreditRatingAxis=cna_AMinusToAPlusPlusMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	cna_CreditRatingAxis=cna_BMinusToBPlusPlusMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type:

X

- Details

Name:	cna_CreditRatingAxis=cna_InsolventMember
Namespace Prefix:
Data Type:	na
Balance Type:
Period Type: