Sales of Businesses - Revenues, costs and expenses, and earnings before income taxes of business disposed (Details) - USD ($) $ in Millions	3 Months Ended		6 Months Ended
	Jun. 30, 2021	Jun. 30, 2020	Jun. 30, 2021	Jun. 30, 2020
Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net earned premiums	$ 1,250	$ 1,184	$ 2,423	$ 2,393
Losses and loss adjustment expenses	714	771	1,381	1,478
Commissions and other underwriting expenses	390	409	770	829
Net investment income	164	88	352	192
Net earnings (loss), including noncontrolling interests	$ 1,002	167	$ 1,421	(137)
Neon Capital Limited
Disclosures by Disposal Groups, Including Discontinued Operations [Line Items]
Net earned premiums		61		132
Losses and loss adjustment expenses		66		106
Commissions and other underwriting expenses		38		70
Underwriting loss		(43)		(44)
Net investment income		0		(6)
Other income and expenses, net		1		(2)
Net earnings (loss), including noncontrolling interests		$ (42)		$ (52)