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, 2014	Sep. 30, 2013	Sep. 30, 2014	Sep. 30, 2013
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (26)	$ (81)	$ 23	$ (112)
Pretax (favorable) unfavorable premium development, excluding Life & Group	3	5	(30)	(16)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(23)	(76)	(7)	(128)
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	(82)	(74)	(139)	(130)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	(3)	(11)	(16)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(84)	(77)	(150)	(146)
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	57	(3)	153	13
Pretax (favorable) unfavorable premium development, excluding Life & Group	(1)	7	(25)	(8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	56	4	128	5
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)	10	5
Pretax (favorable) unfavorable premium development, excluding Life & Group	6	1	6	7
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	6	(5)	16	12
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	(1)	2	(1)
Pretax (favorable) unfavorable premium development, excluding Life & Group				1
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ (1)	$ 2	$ (1)	$ 1