Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2015	Dec. 31, 2014	Dec. 31, 2013
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts, life insurance in force	$ 91,451	$ 88,045	$ 85,015
Gross amounts	4,667	4,468	4,151
Ceded amounts to other companies, life insurance in force	38,716	37,689	36,952
Ceded amounts to other companies	216	237	260
Assumed amounts from other companies, life insurance in force	0	0	0
Assumed amounts from other companies	29	12	11
Net amounts, life insurance in force	52,735	50,356	48,063
Premiums Earned, Net	$ 4,480	$ 4,243	$ 3,902
Percentage of amounts assumed to net, life insurance in force	0.00%	0.00%	0.00%
Percentage of amounts assumed to net	0.70%	0.50%	0.50%
Commercial Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 3,088	$ 2,973	$ 2,777
Ceded amounts to other companies	102	128	151
Assumed amounts from other companies	10	11	10
Premiums Earned, Net	$ 2,996	$ 2,856	$ 2,636
Percentage of amounts assumed to net	0.30%	0.40%	0.40%
Personal Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 1,131	$ 1,080	$ 1,002
Ceded amounts to other companies	35	40	42
Assumed amounts from other companies	1	1	1
Premiums Earned, Net	$ 1,097	$ 1,041	$ 961
Percentage of amounts assumed to net	0.10%	0.10%	0.10%
Excess And Surplus Lines Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 177	$ 156	$ 124
Ceded amounts to other companies	9	8	8
Assumed amounts from other companies	0	0	0
Premiums Earned, Net	$ 168	$ 148	$ 116
Percentage of amounts assumed to net	0.00%	0.00%	0.00%
Reinsurance assumed and other non segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 0	$ 0	$ 0
Ceded amounts to other companies	8	0	0
Assumed amounts from other companies	18	0	0
Premiums Earned, Net	$ 10	$ 0	$ 0
Percentage of amounts assumed to net	188.00%	0.00%	0.00%
Property, Liability And Casualty Insurance Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 4,396	$ 4,209	$ 3,903
Ceded amounts to other companies	154	176	201
Assumed amounts from other companies	29	12	11
Premiums Earned, Net	$ 4,271	$ 4,045	$ 3,713
Percentage of amounts assumed to net	0.70%	0.50%	0.50%
Life Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 271	$ 259	$ 248
Ceded amounts to other companies	62	61	59
Assumed amounts from other companies	0	0	0
Premiums Earned, Net	$ 209	$ 198	$ 189
Percentage of amounts assumed to net	0.00%	0.00%	0.00%