Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (205)	$ (423)	$ (637)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(46)	(8)	43
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(251)	(431)	(594)
CNA Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(135)	(217)	(341)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(15)	(28)	(3)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(150)	(245)	(344)
CNA Commercial [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(46)	(204)	(304)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(35)	21	48
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(81)	(183)	(256)
Hardy [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(11)
Pretax (favorable) unfavorable premium development, excluding Life & Group	3
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(8)
Corporate and Other [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(13)	(2)	8
Pretax (favorable) unfavorable premium development, excluding Life & Group	1	(1)	(2)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ (12)	$ (3)	$ 6