Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions		3 Months Ended								12 Months Ended
		Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2018	Dec. 31, 2017	Dec. 31, 2016
Segment Reporting Information [Line Items]
Total premiums earned										$ 4,865	$ 4,579	$ 4,328
Net investment income										2,094	1,831	1,696
Other income										199	206	224
Revenues before realized gains (losses)										7,416	6,860	6,477
Realized gains (losses) on securities										(266)	5	19
Realized gains on subsidiaries										0	0	2
Total revenues		$ 1,690	$ 2,008	$ 1,833	$ 1,619	$ 1,808	$ 1,835	$ 1,646	$ 1,576	7,150	6,865	6,498
Property and casualty insurance
Segment Reporting Information [Line Items]
Total premiums earned										4,865	4,579	4,328
Net investment income										438	362	350
Other income	[1]									10	28	51
Revenues before realized gains (losses)										5,313	4,969	4,729
Annuity
Segment Reporting Information [Line Items]
Net investment income										1,638	1,458	1,356
Other income										107	103	103
Revenues before realized gains (losses)										1,745	1,561	1,459
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)										358	330	289
Property and transportation | Property and casualty insurance
Segment Reporting Information [Line Items]
Total premiums earned										1,729	1,711	1,662
Specialty casualty | Property and casualty insurance
Segment Reporting Information [Line Items]
Total premiums earned										2,403	2,186	2,006
Specialty financial | Property and casualty insurance
Segment Reporting Information [Line Items]
Total premiums earned										598	576	557
Other specialty | Property and casualty insurance
Segment Reporting Information [Line Items]
Total premiums earned										$ 135	$ 106	$ 103

[1] Includes income of $32 million (before noncontrolling interest) from the sale of an apartment property in 2016.