Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Segment Reporting Information [Line Items]
Net earned premiums									$ 5,099	$ 5,185	$ 4,865
Net investment income									2,132	2,303	2,094
Other income									204	223	223
Revenues before realized gains (losses)									7,597	7,950	7,416
Realized gains (losses) on securities									289	287	(266)
Realized gains on subsidiaries									23	0	0
Total revenues	$ 2,623	$ 2,060	$ 1,951	$ 1,275	$ 2,130	$ 2,123	$ 1,960	$ 2,024	7,909	8,237	7,150
Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums									5,099	5,185	4,865
Net investment income									399	472	438
Other income									8	11	10
Revenues before realized gains (losses)									5,506	5,668	5,313
Annuity
Segment Reporting Information [Line Items]
Net investment income									1,699	1,792	1,638
Other income									131	120	125
Revenues before realized gains (losses)									1,830	1,912	1,763
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)									261	370	340
Property and transportation | Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums									1,871	1,828	1,729
Specialty casualty | Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums									2,235	2,597	2,403
Specialty financial | Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums									613	610	598
Other specialty | Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums									180	150	135
Other lines | Property and casualty insurance
Segment Reporting Information [Line Items]
Net earned premiums									$ 200	$ 0	$ 0