Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended
	Mar. 31, 2021	Mar. 31, 2020
Segment Reporting Information [Line Items]
Net earned premiums	$ 1,173	$ 1,209
Net investment income	188	104
Other income	23	24
Revenues before realized gains (losses)	1,432	1,383
Realized gains (losses) on securities	77	(328)
Total revenues	1,509	1,055
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	1,173	1,209
Net investment income	159	93
Other income	4	5
Revenues before realized gains (losses)	1,336	1,307
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	96	76
Specialty Property and transportation | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	394	386
Specialty casualty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	571	556
Specialty financial | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	157	156
Other specialty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	51	40
Other lines | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	$ 0	$ 71