Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended
	Mar. 31, 2014	Mar. 31, 2013
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ 25	$ (26)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(31)	(13)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(6)	(39)
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	(2)	(15)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(8)	(8)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(10)	(23)
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	17	(11)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(19)	(10)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(2)	(21)
Hardy [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	10	(1)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(4)	4
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	6	3
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
Pretax (favorable) unfavorable premium development, excluding Life & Group		1
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group		$ 2