Segments of Operations - Revenues by segment and sub-segment (Details) - USD ($) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2021	Sep. 30, 2020	Sep. 30, 2021	Sep. 30, 2020
Segment Reporting Information [Line Items]
Net earned premiums	$ 1,529	$ 1,381	$ 3,952	$ 3,774
Net investment income	169	122	521	314
Other income	27	19	70	62
Revenues before realized gains (losses)	1,771	1,563	4,687	4,283
Realized gains (losses) on securities	(17)	23	103	(197)
Realized gains (losses) on subsidiaries	0	(30)	4	(30)
Total revenues	1,754	1,556	4,794	4,056
Real estate-related investments | Continuing operations
Segment Reporting Information [Line Items]
Net investment income		10	51	29
Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	1,529	1,381	3,952	3,774
Net investment income	165	112	467	277
Other income	4	0	9	8
Revenues before realized gains (losses)	1,698	1,493	4,428	4,059
Other
Segment Reporting Information [Line Items]
Revenues before realized gains (losses)	73	60	208	195
Other | Real estate-related investments | Continuing operations
Segment Reporting Information [Line Items]
Net investment income	0	10	51	29
Specialty Property and transportation | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	700	574	1,547	1,350
Specialty casualty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	613	560	1,772	1,663
Specialty financial | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	163	155	477	455
Other specialty | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	53	50	156	132
Other lines | Property and Casualty Insurance
Segment Reporting Information [Line Items]
Net earned premiums	$ 0	$ 42	$ 0	$ 174