Reinsurance (Detail) (USD $) In Millions, unless otherwise specified	12 Months Ended
	Dec. 31, 2011	Dec. 31, 2010	Dec. 31, 2009
Life Insurance in Force
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 77,691	$ 74,123	$ 69,814
Ceded amounts to other companies	35,690	35,016	34,232
Assumed amounts from other companies	1	1	1
Net amounts	42,001	39,108	35,583
Percentage of amounts assumed to net	0.00%	0.00%	0.00%
Earned Premiums
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	3,456	3,291	3,263
Ceded amounts to other companies	274	219	222
Assumed amounts from other companies	12	10	13
Net amounts	3,194	3,082	3,054
Percentage of amounts assumed to net	0.40%	0.40%	0.40%
Earned Premiums | Property, Liability and Casualty Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	3,236	3,080	3,067
Ceded amounts to other companies	219	166	169
Assumed amounts from other companies	12	10	13
Net amounts	3,029	2,924	2,911
Percentage of amounts assumed to net	0.40%	0.40%	0.40%
Earned Premiums | Property, Liability and Casualty Insurance Segment | Commercial Lines
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	2,348	2,281	2,324
Ceded amounts to other companies	162	136	137
Assumed amounts from other companies	11	9	12
Net amounts	2,197	2,154	2,199
Percentage of amounts assumed to net	0.50%	0.40%	0.50%
Earned Premiums | Property, Liability and Casualty Insurance Segment | Personal Lines
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	812	746	715
Ceded amounts to other companies	51	26	31
Assumed amounts from other companies	1	1	1
Net amounts	762	721	685
Percentage of amounts assumed to net	0.20%	0.20%	0.20%
Earned Premiums | Property, Liability and Casualty Insurance Segment | Excess And Surplus Lines Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	76	53	28
Ceded amounts to other companies	6	4	1
Net amounts	70	49	27
Percentage of amounts assumed to net	0.00%	0.00%	0.00%
Earned Premiums | Life Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	220	211	196
Ceded amounts to other companies	55	53	53
Net amounts	$ 165	$ 158	$ 143
Percentage of amounts assumed to net	0.00%	0.00%	0.00%