Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2016	Dec. 31, 2015	Dec. 31, 2014
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts, life insurance in force	$ 95,533	$ 91,451	$ 88,045
Gross amounts	4,857	4,667	4,468
Ceded amounts to other companies, life insurance in force	38,724	38,716	37,689
Ceded amounts to other companies	224	216	237
Assumed amounts from other companies, life insurance in force	0	0	0
Assumed amounts from other companies	77	29	12
Net amounts, life insurance in force	56,808	52,735	50,356
Premiums Earned, Net	$ 4,710	$ 4,480	$ 4,243
Percentage of amounts assumed to net, life insurance in force	0.00%	0.00%	0.00%
Percentage of amounts assumed to net	1.60%	0.70%	0.50%
Commercial Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 3,180	$ 3,088	$ 2,973
Ceded amounts to other companies	98	102	128
Assumed amounts from other companies	7	10	11
Premiums Earned, Net	$ 3,089	$ 2,996	$ 2,856
Percentage of amounts assumed to net	0.20%	0.30%	0.40%
Personal Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 1,195	$ 1,131	$ 1,080
Ceded amounts to other companies	35	35	40
Assumed amounts from other companies	1	1	1
Premiums Earned, Net	$ 1,161	$ 1,097	$ 1,041
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	$ 192	$ 177	$ 156
Ceded amounts to other companies	9	9	8
Assumed amounts from other companies	0	0	0
Premiums Earned, Net	$ 183	$ 168	$ 148
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	20	8	0
Assumed amounts from other companies	69	18	0
Premiums Earned, Net	$ 49	$ 10	$ 0
Percentage of amounts assumed to net	140.80%	188.00%	0.00%
Property, Liability And Casualty Insurance Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 4,567	$ 4,396	$ 4,209
Ceded amounts to other companies	162	154	176
Assumed amounts from other companies	77	29	12
Premiums Earned, Net	$ 4,482	$ 4,271	$ 4,045
Percentage of amounts assumed to net	1.70%	0.70%	0.50%
Life Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 290	$ 271	$ 259
Ceded amounts to other companies	62	62	61
Assumed amounts from other companies	0	0	0
Premiums Earned, Net	$ 228	$ 209	$ 198
Percentage of amounts assumed to net	0.00%	0.00%	0.00%