Claim and Claim Adjustment Expense Reserves (Details of Development Recorded for CNA Commercial Segment) (Detail) (CNA Commercial [Member], USD $) In Millions, unless otherwise specified	3 Months Ended	9 Months Ended
Sep. 30, 2012	Sep. 30, 2011	Sep. 30, 2012	Sep. 30, 2011
CNA Commercial [Member]
Claim And Claim Adjustment Expense Reserves [Line Items]
Commercial auto	$ 9	$ (2)	$ 11	$ (36)
General liability	(21)	4	(26)	26
Workers' compensation	24	3	13	39
Property and other	(10)	(47)	(23)	(128)
Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ 2	$ (42)	$ (25)	$ (99)

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Liability For Other Than Asbestos And Environmental Claims Net Incurred Loss

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Net Prior Year Claim and Allocated Claim Adjustment, Commercial Auto

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Net Prior Year Claim and Allocated Claim Adjustment, General Liability

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Net Prior Year Claim and Allocated Claim Adjustment, Property and Other

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Net Prior Year Claim and Allocated Claim Adjustment, Workers Compensation

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