Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2013	Mar. 31, 2012
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (26)	$ (18)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(13)	(25)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(39)	(43)
CNA Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(15)	(6)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(8)	(9)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(23)	(15)
CNA Commercial [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(11)	(14)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(10)	(17)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(21)	(31)
Hardy [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(1)
Pretax (favorable) unfavorable premium development, excluding Life & Group	4
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	3
Corporate and Other [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	1	2
Pretax (favorable) unfavorable premium development, excluding Life & Group	1	1
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ 2	$ 3