Claim and Claim Adjustment Expense Reserves - Net Prior Year Development (Detail) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 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	$ 24	$ (5)	$ 49	$ (31)
Pretax (favorable) unfavorable premium development	(2)	(8)	(33)	(21)
Total pretax (favorable) unfavorable net prior year development	22	(13)	16	(52)
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	(55)	(41)	(57)	(56)
Pretax (favorable) unfavorable premium development	(1)	(5)	(9)	(13)
Total pretax (favorable) unfavorable net prior year development	(56)	(46)	(66)	(69)
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	79	27	96	16
Pretax (favorable) unfavorable premium development	(5)	(5)	(24)	(15)
Total pretax (favorable) unfavorable net prior year development	74	22	72	1
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		9	10	9
Pretax (favorable) unfavorable premium development	4	2		7
Total pretax (favorable) unfavorable net prior year development	$ 4	$ 11	$ 10	$ 16