Claim and Claim Adjustment Expense Reserves - Net Prior Year Claim and Allocated Claim Adjustment Expense Reserve Development (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
CNA Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Medical professional liability	$ (35)	$ (32)	$ (92)
Other professional liability and management liability	(101)	(22)	(78)
Surety	(74)	(63)	(47)
Warranty	(3)	(5)	(13)
Other	(17)	(13)	13
Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(230)	(135)	(217)
CNA Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Commercial auto	15	27	(98)
General liability	59	(64)	(39)
Workers' compensation	92	15	36
Property and other	(62)	(24)	(103)
Total pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ 104	$ (46)	$ (204)