Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions		3 Months Ended		6 Months Ended
		Jun. 30, 2018	Jun. 30, 2017	Jun. 30, 2018	Jun. 30, 2017
Segment Reporting Information [Line Items]
Total premiums earned		$ 1,161	$ 1,065	$ 2,268	$ 2,087
Net investment income		530	460	1,025	895
Other income		43	47	92	106
Revenues before realized gains (losses)		1,802	1,638	3,514	3,211
Realized gains (losses) on securities		31	8	(62)	11
Total revenues		1,833	1,646	3,452	3,222
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned		1,161	1,065	2,268	2,087
Net investment income		115	96	215	182
Other income	[1]	2	4	4	20
Revenues before realized gains (losses)		1,278	1,165	2,487	2,289
Annuity
Segment Reporting Information [Line Items]
Net investment income		412	360	806	707
Other income		27	26	53	53
Revenues before realized gains (losses)		439	386	859	760
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)		85	87	168	162
Specialty Property and transportation | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned		374	357	724	699
Specialty casualty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned		595	537	1,174	1,045
Specialty financial | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned		159	146	308	293
Other specialty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Total premiums earned		$ 33	$ 25	$ 62	$ 50

[1] Includes income of $13 million (before noncontrolling interest) from the sale of a hotel in the first quarter of 2017.