Claim and Claim Adjustment Expense Reserves - Net Prior Year Development (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	$ (26)	$ (81)	$ 23	$ (112)
Pretax (favorable) unfavorable premium development	3	5	(30)	(16)
Total pretax (favorable) unfavorable net prior year development	(23)	(76)	(7)	(128)
CNA Specialty [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(82)	(74)	(139)	(130)
Pretax (favorable) unfavorable premium development	(2)	(3)	(11)	(16)
Total pretax (favorable) unfavorable net prior year development	(84)	(77)	(150)	(146)
CNA Commercial [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	57	(3)	153	13
Pretax (favorable) unfavorable premium development	(1)	7	(25)	(8)
Total pretax (favorable) unfavorable net prior year development	56	4	128	5
Other Insurance [Member]
Causes of Increase (Decrease) in Liability for Unpaid Claims and Claims Adjustment Expense [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development	(1)	(4)	9	5
Pretax (favorable) unfavorable premium development	6	1	6	8
Total pretax (favorable) unfavorable net prior year development	$ 5	$ (3)	$ 15	$ 13