Claim and Claim Adjustment Expense Reserves (Net prior year development) (Details) (USD $) In Millions, unless otherwise specified	3 Months Ended		6 Months Ended
	Jun. 30, 2014	Jun. 30, 2013	Jun. 30, 2014	Jun. 30, 2013
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ 24	$ (5)	$ 49	$ (31)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(2)	(8)	(33)	(21)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	22	(13)	16	(52)
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	(55)	(41)	(57)	(56)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(1)	(5)	(9)	(13)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(56)	(46)	(66)	(69)
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	79	27	96	16
Pretax (favorable) unfavorable premium development, excluding Life & Group	(5)	(5)	(24)	(15)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	74	22	72	1
Hardy [Member]
Segment Reporting Information [Line Items]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group		12	10	11
Pretax (favorable) unfavorable premium development, excluding Life & Group	4	2		6
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	4	14	10	17
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		(3)		(2)
Pretax (favorable) unfavorable premium development, excluding Life & Group				1
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group		$ (3)		$ (1)