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	9 Months Ended
Sep. 30, 2015	Sep. 30, 2014	Sep. 30, 2015	Sep. 30, 2014
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (175)	$ (26)	$ (187)	$ 23
Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(130)	(79)	(141)	(123)
Medical Professional Liability	(19)	16	(11)	17
Other Professional Liability and Management Liability	(37)	(9)	(41)	(59)
Surety	(70)	(79)	(69)	(78)
Warranty	0	0	1	0
Other	$ (4)	$ (7)	$ (21)	$ (3)

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
Balance Type:	debit
Period Type:	duration

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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