Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Segment Reporting Information [Line Items]
Net earned premiums	$ 1,250	$ 1,184	$ 2,423	$ 2,393
Net investment income	164	88	352	192
Other income	20	19	43	43
Revenues before realized gains (losses)	1,484	1,337	2,916	2,720
Realized gains (losses) on securities	43	108	120	(220)
Realized gain on subsidiary	4	0	4	0
Total revenues	1,531	1,445	3,040	2,500
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	1,250	1,184	2,423	2,393
Net investment income	143	72	302	165
Other income	1	3	5	8
Revenues before realized gains (losses)	1,394	1,259	2,730	2,566
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	90	78	186	154
Specialty Property and transportation | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	453	390	847	776
Specialty casualty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	588	547	1,159	1,103
Specialty financial | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	157	144	314	300
Other specialty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	52	42	103	82
Other lines | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	$ 0	$ 61	$ 0	$ 132