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, 2013	Jun. 30, 2012	Jun. 30, 2013	Jun. 30, 2012
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	$ (5)	$ (52)	$ (31)	$ (70)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(8)	(23)	(21)	(48)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(13)	(75)	(52)	(118)
CNA Specialty [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(41)	(35)	(56)	(41)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(5)	(5)	(13)	(14)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	(46)	(40)	(69)	(55)
CNA Commercial [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	27	(13)	16	(27)
Pretax (favorable) unfavorable premium development, excluding Life & Group	(5)	(19)	(15)	(36)
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	22	(32)	1	(63)
Hardy [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	12		11
Pretax (favorable) unfavorable premium development, excluding Life & Group	2		6
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	14		17
Corporate and Other [Member]
Pretax (favorable) unfavorable net prior year claim and allocated claim adjustment expense reserve development, excluding Life & Group	(3)	(4)	(2)	(2)
Pretax (favorable) unfavorable premium development, excluding Life & Group		1	1	2
Total pretax (favorable) unfavorable net prior year development, excluding Life & Group	$ (3)	$ (3)	$ (1)