Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2012	Mar. 31, 2011
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (18)	$ (19)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(25)	(16)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(43)	(35)
CNA Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(6)	(15)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(9)	(7)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(15)	(22)
CNA Commercial [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(14)	(7)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(17)	(8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(31)	(15)
Corporate and Other Non-Core [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	2	3
Pretax (favorable) unfavorable premium development, excluding Life & Group	1	(1)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ 3	$ 2