Claim and Claim Adjustment Expense Reserves (Net prior year claim and allocated claim adjustment expense reserve development for Specialty segment) (Details) - USD ($) $ in Millions	3 Months Ended	6 Months Ended
Jun. 30, 2015	Jun. 30, 2014	Jun. 30, 2015	Jun. 30, 2014
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (5)	$ 24	$ (12)	$ 49
Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(13)	(41)	(11)	(44)
Medical Professional Liability	(6)	1	8	1
Other Professional Liability and Management Liability	(1)	(44)	(4)	(50)
Surety	0	0	1	1
Warranty	1	0	1	0
Other	$ (7)	$ 2	$ (17)	$ 4

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

No definition available.

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Period Type:	duration

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

No definition available.

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Net prior year claim and allocated claim adjustment expense reserve development, Surety

No definition available.

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Net prior year claim and allocated claim adjustment expense reserve development, Warranty

No definition available.

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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
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Net prior year claim and allocated claim adjustment expense reserve development, Other Professional Liability and Management Liability

No definition available.

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Namespace Prefix:	cna_
Data Type:	xbrli:monetaryItemType
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Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group

No definition available.

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

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