Claim and Claim Adjustment Expense Reserves Net prior year claim and allocated claim adjustment expense reserve development for International 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
International [Member]
Medical Professional Liability	(8)	(3)	(8)	(2)
Other Professional Liability	(11)	1	(16)	(14)
Liability	(5)	(3)	(12)	(9)
Property & Marine	(5)	(11)	(19)	(10)
Other	(5)	(1)	9	(7)
Commutations	0	0	0	10
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (34)	$ (17)	$ (46)	$ (32)

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

No definition available.

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

No definition available.

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Data Type:	xbrli:monetaryItemType
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Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Commutations

No definition available.

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Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Liability

No definition available.

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Namespace Prefix:	cna_
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Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Other Professional Liability

No definition available.

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Balance Type:	debit
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Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development, Property & Marine

No definition available.

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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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Data Type:	xbrli:monetaryItemType
Balance Type:	debit
Period Type:	duration

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