Reinsurance (Details) - USD ($) $ in Millions	12 Months Ended
	Dec. 31, 2017	Dec. 31, 2016	Dec. 31, 2015
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts, life insurance in force	$ 99,888	$ 95,533	$ 91,451
Gross amounts	5,052	4,857	4,667
Ceded amounts to other companies, life insurance in force	38,711	38,724	38,716
Ceded amounts to other companies	230	224	216
Assumed amounts from other companies, life insurance in force	0	0	0
Assumed amounts from other companies	132	77	29
Net amounts, life insurance in force	61,177	56,808	52,735
Premiums Earned, Net	$ 4,954	$ 4,710	$ 4,480
Percentage of amounts assumed to net, life insurance in force	0.00%	0.00%	0.00%
Percentage of amounts assumed to net	2.70%	1.60%	0.70%
Commercial Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 3,258	$ 3,180	$ 3,088
Ceded amounts to other companies	99	98	102
Assumed amounts from other companies	6	7	10
Premiums Earned, Net	$ 3,165	$ 3,089	$ 2,996
Percentage of amounts assumed to net	0.20%	0.20%	0.30%
Personal Insurance
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 1,275	$ 1,195	$ 1,131
Ceded amounts to other companies	35	35	35
Assumed amounts from other companies	1	1	1
Premiums Earned, Net	$ 1,241	$ 1,161	$ 1,097
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	$ 219	$ 192	$ 177
Ceded amounts to other companies	10	9	9
Assumed amounts from other companies	0	0	0
Premiums Earned, Net	$ 209	$ 183	$ 168
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	18	20	8
Assumed amounts from other companies	125	69	18
Premiums Earned, Net	$ 107	$ 49	$ 10
Percentage of amounts assumed to net	116.10%	140.80%	188.00%
Property, Liability And Casualty Insurance Segment [Member]
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 4,752	$ 4,567	$ 4,396
Ceded amounts to other companies	162	162	154
Assumed amounts from other companies	132	77	29
Premiums Earned, Net	$ 4,722	$ 4,482	$ 4,271
Percentage of amounts assumed to net	2.80%	1.70%	0.70%
Life Insurance Segment
Reinsurance Premiums for Insurance Companies, by Product Segment [Line Items]
Gross amounts	$ 300	$ 290	$ 271
Ceded amounts to other companies	68	62	62
Assumed amounts from other companies	0	0	0
Premiums Earned, Net	$ 232	$ 228	$ 209
Percentage of amounts assumed to net	0.00%	0.00%	0.00%