Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2013	Dec. 31, 2012	Dec. 31, 2011
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (118)	$ (205)	$ (423)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(42)	(46)	(8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(160)	(251)	(431)
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	(230)	(135)	(217)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(17)	(15)	(28)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(247)	(150)	(245)
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	104	(46)	(204)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(9)	(35)	21
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	95	(81)	(183)
Hardy [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	14	(11)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(17)	3
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(3)	(8)
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	(6)	(13)	(2)
Pretax (favorable) unfavorable premium development, excluding Life & Group	1	1	(1)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ (5)	$ (12)	$ (3)