Claim and Claim Adjustment Expense Reserves Net prior year claim and allocated claim adjustment expense reserve development for International segment (Details) - USD ($) $ in Millions	12 Months Ended
Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (210)	$ (21)	$ (118)
International [Member]
Medical Professional Liability	(9)	(7)	(7)
Other Professional Liability	(16)	(26)	(30)
Liability	(17)	(13)	(8)
Property & Marine	(29)	(14)	13
Other	17	(9)	(17)
Commutations	0	10	11
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (54)	$ (59)	$ (38)

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

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