Reinsurance (Details) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2012	Dec. 31, 2011	Dec. 31, 2010
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 3,755	$ 3,456	$ 3,291
Ceded amounts to other companies	242	274	219
Assumed amounts from other companies	9	12	10
Net amounts	3,522	3,194	3,082
Percentage of amounts assumed to net	0.30%	0.40%	0.40%
Commercial Lines [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	2,524	2,348	2,281
Ceded amounts to other companies	149	162	136
Assumed amounts from other companies	8	11	9
Net amounts	2,383	2,197	2,154
Percentage of amounts assumed to net	0.30%	0.50%	0.40%
Personal Lines [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	897	812	746
Ceded amounts to other companies	30	51	26
Assumed amounts from other companies	1	1	1
Net amounts	868	762	721
Percentage of amounts assumed to net	0.10%	0.20%	0.20%
Excess And Surplus Lines Insurance [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	99	76	53
Ceded amounts to other companies	6	6	4
Net amounts	93	70	49
Percentage of amounts assumed to net	0.00%	0.00%	0.00%
Property, Liability And Casualty Insurance Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	3,520	3,236	3,080
Ceded amounts to other companies	185	219	166
Assumed amounts from other companies	9	12	10
Net amounts	3,344	3,029	2,924
Percentage of amounts assumed to net	0.30%	0.40%	0.40%
Life Insurance Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	235	220	211
Ceded amounts to other companies	57	55	53
Net amounts	178	165	158
Percentage of amounts assumed to net	0.00%	0.00%	0.00%
Life Insurance in Force
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	81,467	77,691	74,123
Ceded amounts to other companies	36,340	35,690	35,016
Assumed amounts from other companies		1	1
Net amounts	$ 45,126	$ 42,001	$ 39,108
Percentage of amounts assumed to net	0.00%	0.00%	0.00%