Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		9 Months Ended
	Sep. 30, 2013	Sep. 30, 2012	Sep. 30, 2013	Sep. 30, 2012
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (81)	$ (40)	$ (112)	$ (110)
Pretax (favorable) unfavorable premium development, excluding Life & Group	5	(7)	(16)	(55)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(76)	(47)	(128)	(165)
CNA Specialty [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(74)	(39)	(130)	(80)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(3)	(1)	(16)	(15)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(77)	(40)	(146)	(95)
CNA Commercial [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(3)	2	13	(25)
Pretax (favorable) unfavorable premium development, excluding Life & Group	7	(5)	(8)	(41)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	4	(3)	5	(66)
Hardy [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(6)	(6)	5	(6)
Pretax (favorable) unfavorable premium development, excluding Life & Group	1		7
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(5)	(6)	12	(6)
Corporate and Other [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	2	3		1
Pretax (favorable) unfavorable premium development, excluding Life & Group		(1)	1	1
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ 2	$ 2	$ 1	$ 2